httroduction to Animal Morphology. I'jc^ 



the two parts are not functional together. The ova 

 completely segment, and produce rotating embryos, 

 developing one, rarely three, ciliary circlets, the front 

 one of which becomes a bilobed velum. This is lost 

 as the foot and epipodia develop. 



There are two orders, including twenty-three genera and 

 ninety living species : — 



1. Thecosomata {BlainvtUe) — having a shell, an indistinct 

 head, gills in the mantle cavity, and a 2-5 rowed radula. 

 Families: — i. Hyaleidas — shells inoperculate, symmetrical, 

 subulate (Styliola), triangular, depressed (Balantiura), inter- 

 nally septate (Triptera), or globular, with a lateral slit 

 (Cavolina). 2. Thecidas — Palseozoic, operculate, gill-less, 

 with straight shell. 3. Limacinidce — operculate, sinistrally 

 spiral, turreted (Heterofusus), or helicoid, open in Eccyliom- 

 phalus. 4. Cymbuliidas — shell internal ; fin not retractile ; 

 gills none, ex. Tiedemannia, Halopsyche. The fossil families, 

 Conularidse and Tentaculitidae, unite these to Hyaleidse. 



2, Gymnosomata {^Blainvilk) — shell and mantle absent ; 

 head and foot distinct ; larva with several ciliary zones. 

 Families: — i. Clionidag — gills none; epipodial lobes one 

 pair ; head with a pair of stalked eyes, three pair of head 

 cones, and two (Clione), four (Clionopsis), or no (Cliodita) 

 tentacles. The foot is rarely absent (Pteropelagia). 2. Pneu- 

 modermonidae have external gills diversely placed as noted 

 above. 



