Anaesthesis

Specimens in seawater were anaesthetised by slowly adding 70% alcohol, and preserved in 5% formaldehyde solution. Serial sections were cut at 10μ on the microtome and stained with Heidenhain's haematoxylin and eosin. Observations of the external features and general behaviour were made on living specimens both in the field and in the laboratory.

External Features

The body is divided into an anterior chaetigerous region including the prostomium, a middle branchial region, and a posterior caudal region (Fig. 1, C).

The anterior region is made up of the prostomium or proboscis (Fig. 1. C, D — Pros.), and six chaetigerous segments — each segment consisting of the chaetigerous annulus (Fig. 1, C, D — Chaet. ann.) together with three preceding annuli, and one following annulus. Evidence for this is based on the position of the diaphragm (Fig. 3 —Dphm. 2 and Dphm. 3).

The proboscis is top-shaped when fully extended and covered by short papillae which extend into the oral zone (Fig. 1, D; Fig. 2, B — Pap.). The proboscis cannot be completely withdrawn as in the genus Arenicola.

The specimen sketched (Fig. 1, C) shows that proboscis in a fairly advanced stage of contraction, while in Fig. 1, D, the proboscis is fully extended. A large nuchal pore (Fig. 1, D — Nuch. p.) is located on the mid-dorsal line just behind the proboscis, and opens into a cavity lined with glandular cells (Fig. 2, B — Nuch. p.). The chaetigerous section possesses six neuropodia (Fig. 1, C, G — Neurop.), appearing as transverse slits extending from beneath the notopodia to within about 2 mm. of the ventral mid-line. Each notopodium bears a double row of notopodial chaetae (Fig. 1G, No. chaet.). These are stiff, bristle-like structures which may be articulated in an anterior-posterior direction so as to aid in movement of the animal. A small hooded nephridiopore (Fig. 1, C — Neph. p.) is located just below the fourth to the ninth notopodium on each side. The epidermis of both the anterior chaetigerous and the branchial regions consists of raised polygonal areas of mucus-secreting columnar cells separated by shallow grooves (Fig. 2, C —Epid.).

The brancial region, in addition to neuropodia. notopodia, and setae, is distinguished by the presence of thirteen pairs of contractile gills (Fig. 1, C— Brl -Brl3), which are attached just behind each notopodium. The front pair of gills is always smaller than the rest but still has the same well-developed feathery structure. Both the page 123 gills and parapodia (notopodium and chaetae) will retract on application of strong light, interference or immersion in a foreign non-anaesthetic medium.

The neuropodia in this region are separated from the strongly marked ventral mid-line (Fig. 1, C — v.m.l.) by a distance roughly equal to the length of the neuropodia themselves (Fig. 1, G).

The caudal region is typified by the absence of any specialised external structures (Fig. 1, C — Tail). The tail is very variable in length. Living specimens were found with twenty to thirty tail segiters, while others had as few as two or three. It is probable that specimens with very short tails have at some time been damaged by predators.

The worm brings its anus to the surface of the sand in order to defaecate, and consequently the tip falls easy prey to flounders, or other bottom feeding fish and to the hundreds of herring gulls patrolling the beach at low water. Many gulls were observed to pounce on defaecating worms and plunge their beaks into the burrow. In general, the tail segments are longest in the region of the anus.

Internal Anatomy

Dissection was performed on a freshly-killed specimen which had been previously relaxed in alcohol so that the pressure of the coelmic fluid would not force the internal organs through the primary incision in the body wall. The body cavity was opened by a mid-dorsal longitudinal incision from the proboscis to a short distance down the tail, and the flaps of the body wall were pinned back (Fig. 3). The gut was slightly displaced to the left and pinned.

The coelom is very spacious, and extends from one end of the body to the other. Anteriorly the coelom is partially transversely divided by the retractor muscles of the proboscis (Fig. 3 — mm. p. ret.) and more completely so by three septa (Fig. 3 — dphm. 1), which arise from the groove behind the first annulus posterior to each of the first three chaetigerous annuli. The septa are perforated to allow passage of the coelomic fluid so, in effect, the continuity of flow of the coelomic fluid is not appreciably interrupted. Dorsal and ventral mesenteries support the oesophagus and anterior portion of the gut from the first to the third septum.

From the third septum to the base of the tail, the body cavity is undivided. No mesenteries support the gut which is slightly larger than the portion of coelom in which it lies, and it tends to swing freely with movements of the body. Segmental blood vessels, however, give some elastic support to the mid-gut. Arrangement of repeating organs such as nephridial funnels (Fig. 3 — Neph. f.) and somatic segmental afferent and efferent blood vessels, indicate the segments in this body region.

From the base of the ‘tail’ region to the anus, transverse septa (Fig. 3 — T. dphm.) divide the body cavity into clearly recognisable segments.

The musculature of the body wall is arranged in an outer circular band (Fig. 2, C — Circ. mm.) and a thick inner longitudinal muscle sheath (Fig. 2, C — Long. mm.) which is divided into one dorsal and two ventrolateral sections by the left and right oblique muscle layers (Fig. 2, C — Obl. mm.). The oblique muscles, which divide the coelom longitudinally into three compartments, commence behind the third septum and lose their identity after the first few ‘tail’ segments.

The retractor muscle of the proboscis (Fig. 2, B — mm. P. ret.) is a very thin, weakly developed modification from the longitudinal muscle layer. The retractor muscles of the parapodia (Fig. 3 — mm. Para. ret.) are also derived from the longitudinal muscle, and they extend almost to the mid-ventral line. The parapodial proctractor muscles (Fig. 3 — mm. Para. prot.) are usually eight in number — four each side of the chaetigerous sac, and these muscles move the parapodium in an antero-posterior direction.

Alimentary System

These caeca or sacs are arranged in two longitudinal rows, one on each side of the oesophagus, and in fresh specimens are red in colour owing to their large blood supply. The anterior pair of caeca are always much larger and longer than the following caeca. The most posterior pairs are in general the smallest of all. One specimen was found to have only eleven pairs of oesophageal caeca and the largest number noted was twenty-one. Figure 3 illustrates a specimen with nineteen pairs of caeca.

FIGURE 3
Abarenicola dissected to show major features of internal anatomy. Ant. oes. caec. — Anterior oesophageal caecum, Br. aff. — Afferent branchial vessel, Br. eff. — Efferent branchial vessel, Br. Nn. — Annulus branch nerve, Chloro. tiss. — Chlorogogenous tissue, Dors. B.V. — Dorsal blood vessel, Dphm. — first 2 anterior septa, Long. mm. — Longitudinal muscle, mm. Para. prof.— Parapodial protractor muscles, mm. Para. ret. — Parapodial retractor muscles, Mth. — Mouth, Neph. f. — Nephridial funnel, (Nephrostome) Neph. sac — Nephridial sac, N.C. — Ventral nerve cord, New. B.V. — Neural blood vessel, Oes caec. — Oesophageal caeca, Para 1 -Para XIX — Parapodia, Phar. — Pharynx, Seg. eff. B.V.— Segmental efferent blood vessel, T. dphm. — Tail septum, Vas. gut. ret. — Vascular reticulum of gut, Vent. B.V. — Ventral blood vessel.

Vascular System

The general features only of the blood system of Abarenicola assimilis are noted here.

There are two chief vessels — one above and one below the alimentary tract, running from one end of the body to the other (Fig. 3). The gut is served by a reticulate system of capillaries in turn which are connected via a pair of red bulbous hearts (Fig. 3 — H.) to a ventral vessel. The dorsal vessel originates near the anus and gives off a pair of segmental vessels (Fig. 3 — Seg. B.V.) at the beginning of each caudal segment. This blood is collected by the ventral vessel. In the gastric region, the blood from the last seven pairs of gills returns by way of afferent vessels to the dorsal vessel, but anteriorly from the level of the sixth gill the dorsal vessel receives numerous branches from the gastric reticulum, but no efferent branchial or nephridial vessels. These are returned via the gastric reticulum. In front of the heart the dorsal vessel receives branches from the oesophagus and each oesophageal caecum, and after many branches are received from the brain it finally disappears in the tiny capillaries of the proboscis.

The ventral vessel is large and covered by chlorogogenous tissue (Fig. 2, C — Chloro. tiss.) and sends efferent branches to the brain, oesophageal caeca, each of the gills, and the caudal septa. It does not appear to have any direct connection with the ventral part of the gut other than through the heart. Other vessels present are lateral vessels supplying the body wall (Fig. 2, C — Lat. B.V.), neural vessels lying each side of the nerve cord along the length of the body (Fig. 2, C; Fig. 3 — Neu. B.V.), and efferent and afferent branchial vessels shown in the section of the gills (Fig. 2, C). The blood flow is: The heart draws blood from the gastric vessels into the highly elastic ventral vessel, from whence it circulates through the body.

Nervous System and Sense Organs

The nervous system consists of the brain (Fig. 2, B — Bra.), the circum-oesophageal commissures (Fig. 2, B — Com.) and the ventral page 127 nerve cord (Figs. 2, B, C — N.C.). The brain is a small, oval-shaped, richly vascularised knot of nervous tissue situated just anterior to the nuchal organ on the dorsal aspect. The circum-oesophageal commissures originate from the brain and form the ventral nerve cord from their ventral convergence. As far as could he traced, the ventral nerve cord gives off a pair of nerves in each annulus of the body, thus giving each segment five pairs of branch nerves (Fig. 3 — Br. Nn.). The ventral nerve cord is not obviously ganglionated.

In close apposition to the brain is the nuchal sense organ (Fig. 1, D; 2, B — Nuch. P.). Dorsally it appears as a dark pit divided by a single median papilla (Fig. 1, D) and in longitudinal section the cells are large and distinct (Fig. 2, B). The parapodia and gills are extremely sensitive to light and touch.

Excretory System

There are six pairs of nephridia in all (Fig. 3 — Neph.) located in the fourth to the ninth chaetigerous segment. They are quite large and readily seen. Each nephridium consists of a funnel opening into the body cavity, an elongate central portion, and a small end sac, which opens to the exterior through the nephridiopore situated just below the notopodium. Each nephridium is supplied with an efferent and afferent blood vessel.

Reproductive System

Gonads are present in conjunction with each of the six pairs of nephridia in the breeding season. The oocytes and spermatoblasts break free to complete their development in the coelomic cavity. On April 1, 1961, a few worms had liberated their gonadial contents into the coelom, and by August 1, 1961. the breeding season was in full swing. Spermatozoa are aggregated in plates up to 0.75 mm. in diameter until fully mature (Fig. 1, F, 1, 2). When fully mature, the individual spermatazoa break free. Each measures about 1/200 in. in length. When spawning, ripe ova surrounded by a thin membrane, or spermatazoa, are drawn by the action of the ciliated nephrostomes into the nephridia. and liberated through the nephridiopores into the seawater, where fertilisation takes place.

At fertilisation (which was achieved in the laboratory) the diameter of a mature ovum was found to be approximately 0.25 mm. (Fig. 1, E). The nucleus of the ovum is conspicuous and spherical and the cytoplasm is very granular.

After fertilisation the zygote shrinks within the fertilisation membrane and then undergoes a spiral system of cleavage to form a morula.