SCAPHOPODA : PTEROPODA. 467 



rudimentary shell-gland, and perhaps a foot-gland. The anus is formed as 

 a proctodaeum. 



There are three genera, Dentalium, Siphonodentalium, and Entalium. 

 The animal lives with the anterior extremity plunged into the sand on the 

 sea coast, at depths of ten to a hundred fathoms. Dentalium occurs fossil 

 in Carboniferous strata. 



Dentalium, de Lacaze Duthiers, A. Sc. N. (4), vi. 1856; vii. and viii. 1857. 

 Development, Kowalewsky, Annales Mus. Nat. Hist. Marseilles, i. 1883. Compa- 

 rison with Cephalopoda, Grobben, Arb. Zool. Inst. Wien, v. 1884 (p. 44 of his 



paper). 



CLASS PTEROPODA. 



Pelagic Glossophora with the median foot much reduced, but with two 

 large lateral pedal lobes (epipodia ?) developed into swimming organs. Vis- 

 ceral dome elongated and secondarily symmetrical \ rarely coiled spirally ; either 

 naked or covered with a shell. No ctenidia ; anus, generative and nephridial 

 apertures placed anteriorly, and ^lsually on the right side of the body. Her- 

 viapJirodite. 



The bilateral symmetry of the Pteropoda is without doubt a symmetry 

 secondarily acquired (p. 455), and in many points, both of development and 

 anatomy, they closely resemble the Gastropoda. The foot is very small in 

 size. In Clio it consists of two pro-meso-podial lobes, and a small meta- 

 podium. The latter bears an operculum in Spirialis and Heterofusus 

 (Limacinidae\ and the Veliger of Cymbulia and Tiedemannia. The two 

 swimming lobes are probably homologous with epipodia, and generally 

 appear later than the other parts of the foot. In the Pteropoda with shells, 

 or Thecosomata, they grow round the head, which is consequently not 

 distinct in this order. In the Gymnosomata, where the adult has no shell, 

 the head is furnished with two pairs of cephalic tentacles an anterior pair, 

 which is not represented in Thecosomata, and a posterior nuchal pair. The 

 latter are eye-bearing in Gymnosomata, in the adult Creseis, and the young 

 Tiedemannia and Spirialis among Thecosomata. The mantle aborts in 

 Gymnosomata ; it forms a deep fold on the ventral aspect in Thecosomata ; 

 but in the Limacinidae, with a spirally twisted visceral dome, the fold is 

 dorsal in position. All Pteropoda are furnished with a larval shell, formed 

 by the everted shell-gland. A secondary shell is added subsequently. 

 Both are thrown off in Gymnosomata and the Cymbulidae ; the larval shell 

 only in Hyaleidae ; while both persist in Creseis (= Styliola). The Cymbu- 

 lidae form a third persistent hyaline shell of cartilaginous consistency. In 

 other Thecosomata the shell is also hyaline, and apparently highly calcified, 



