Introduction to A nimal Morphology. 279 



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 (Blainville) having a shell, an indistinct 

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

 Families: i. Hyaleidse shells inoperculate, symmetrical, 

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

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

 (Cavolina). 2. Thecidoe Palaeozoic, operculate, gill-less, 

 with straight shell. 3. Limacinidse operculate, sinistrally 

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

 phalus. 4. Cymbuliidae shell internal ; fin not retractile ; 

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

 Conularidae and Tentaculitidae, unite these to Hyaleidae. 



2. Gymnosomata {Blaiircilh} shell and mantle absent ; 

 head and foot distinct ; larva with several ciliary zones. 

 Families: i. Clionidae 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- 

 modermonidag have external gills diversely placed as noted 

 above. 



