Heteropods are marine free-swimming gastropods, and relatively common members of the plankton in tropical and subtropical seas, particularly in the Indo-Pacific region. Occasional specimens may be stranded on the shore, otherwise they are found only in plankton tows. The body is adapted to pelagic life, the familiar creeping gastropod foot being modified as a more or less single, fin-like swimming organ, unlike the Pteropoda where the foot is developed as two anterior symmetrical lobes. The spiral shell may be present, well-formed, reduced, or even absent. Species of the F. Atlantidae are the least adapted to pelagic life, having a spiralled shell containing the visceral mass like most other gastropods, and a flattened, elongate swimming organ developed from the middle and posterior portions of the foot. The F. Atlantidae are also the only heteropods in which the body can be completely withdrawn into the shell, and the posterior region of the foot may carry an operculum. The shell is flattened and rather discoid with the outer whorl showing a pronounced keel. In the F. Carinariidae the shell is greatly reduced, helmet-shaped, and covers only the stalked visceral hump, or a portion of it, about the mid-region of the trunk. ‘Head’ and ‘tail’ regions are well formed (Fig. 1, A), much larger than in the F. Atlantidae and cannot be withdrawn into the shell. Lastly, there is the F. Pterotracheidae in which the shell is entirely lacking, and the small, compact visceral mass is situated in the posterior third of a more or less cylindrical, streamlined body (Fig. 1, F). Species of this family are the best adapted to pelagic life. In both the Carinariidae and Pterotracheidae the foot is relatively small compared with the body, very much compressed and hatchet-shaped.

A particularly conspicuous feature of species of the F. Pterotracheidae is the prominent telescopic eyes, and anyone who is familiar with Zahl's excellent photograph of a living Pterotracheae coronata from the Mediterranean (Nat. Geog. Mag. 1953, Vol. CIV (5)), will agree with his name for this animal — ‘the mollusc with the tear drop eyes’. Unfortunately the eyes in preserved specimens are usually contracted back into the head.

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Heteropods are carnivorous and known to be voracious predators of jellyfish, comb jellies, salps, fish larvae, copepods, etc.

The foot is uppermost when a heteropod is swimming, and this is another of their characteristics which makes its difficult to realise that they are closely allied to the creeping common shore periwinkle Littorina. The figures in the present paper show the foot facing downwards because this enables ready comparison with illustrations of other members of the Gastropoda in the literature.

Over the period of a few years, five small heteropods, representing two species, have been found among collections of coelenterates sent to the author. The transparency of the greater part of the body and often also the shell, displays clearly the internal structure, and led to further interest in the group and so to the identification of the above material as well as a larger specimen in the teaching collection of the Zoology Department, Victoria University of Wellington. As descriptions and illustrations of New Zealand heteropods are not readily available, the species in the present collection are illustrated in some detail. They show clearly the characters of the F. Carinariidae and F. Pterotracheidae. Only species of these families are at present recorded from New Zealand coastal waters. A key is also given to assist in recognition of all the known New Zealand species.

Benham's (1905) account of a single damaged specimen of Pterotrachea coronata Forskal washed ashore at Long Beach, Otago, was the only description of a heteropod taken round our coasts until Tesch recorded five species, four of them new to these waters, nearly forty-five years later, in 1949. There are also, however, records of heteropods from open ocean waters, and Carinaria lamarcki was described by Quoy and Gaimard under the name of C. australis as early as 1832 from a locality in the Tasman Sea about halfway between Bass Strait and Cook Strait. Tesch remarks that Firoloida desmaresti ‘swarm (s) everywhere in the Tasman Sea, but disappears suddenly at the South Island of New Zealand (like all Heteropods even Atlanta)’.

Carinaria lamarcki and Firoloida desmaresti are the only two species in my collection. F. desmaresti was taken in plankton tows off Auckland and in Cook Strait; C. lamarcki was taken in Wellington Harbour. As noted above, heteropods live in warm water and it seems likely that a search through other New Zealand subtropical plankton collections and further netting in appropriate localities will show that heteropods are among the common surface water animals of our coasts. Subtropical water may occur seasonally in coastal regions as far south as Otago and the Chatham Islands (Fleming, 1944) and Benham's record of Pterotrachea coronata from Long Beach, Otago, is good evidence for this opinion.

In the present collection Carinaria lamarcki is represented only by a very young specimen 5.0 mm. in length, and Firoloida desmaresti by four specimens 5.0 mm. to 10.0 mm. in length taken off Auckland, and a much larger one 75.0 mm. in length from Cook Strait. In specimens of F. desmaresti 10.0 mm. in length and longer, all the major body regions page 118 of proboscis, foot, visceral mass, etc., can be seen with the naked eye, and to some extent also the buccal bulb, supra-oesophageal ganglionic mass, eyes, pedal ganglia and the nerves radiating from them. Microscopic examination is necessary to distinguish details of the alimentary and nervous systems and the cuticular spots. A distinct downward flexure of the proboscis toward the side on which the foot is situated is seen in young animals of F. desmaresti (Fig. 1, C) but the flexure is not evident in the larger 75.0 mm. specimen (Fig. 1, F).

The shell is missing in my specimen of C. lamarcki but the outline of the shell keel can be traced in the dorsal region of the soft tissues of the visceral hump (Fig. 1, A, ke). So far I have been unable to obtain a shell of this species, and Fig. 1, B, shows the shell re-drawn from Tesch (1949).

All the specimens in the present collections appear to be females, the penis being absent from the right side in the specimen of C. lamarcki, while in the specimens of F. desmaresti there are no tentacles before the eyes and no sucker on the foot. These are female characters. Some specimens do not have the egg-carrying caudal peduncle.

Key to New Zealand Heteropoda

As recognised at present, the g. Firoloida is monotypic with F. desmaresti Lesueur, 1817, the only known species (Fig. 1, C-F). Males of this species have tentacles in front of the eyes and a sucker on the foot. Both these structures are absent in the female.)