Introduction to Animal Morphology. 303 



larvae (nucleus, p. 285) differ considerably in some cases from 

 those of the adults. In Opisthobranchs the larval shell and 

 mantle often disappear; inMarseniathe larval shell (Echino- 

 spira) is lost, and a new one developed.* In the embryos of 

 some slugs there is a pulsating caudal sac (in Limnxa two 

 sacs), which seems to keep up a circulation ; this is lost in the 

 adult. In Pulmonates the larva has no conspicuous velum, 

 but a homologous structure . and often persists in 



the adult, as in Limnxa ; on the left side of the neck in this 

 genus a primitive /"-shaped kidney of round cells and con- 

 nective tissue is developed. They have also a cervical um- 

 bilical vesicle of stellate and fusiform cells and fibres. Some 

 arc very prolific. Dendronotus Ascanii has been calculated 

 to lay 25,000 eggs annually, and a S. Atlantic Doris 600,000. 



They are mostly marine. A few are fixed ; but most 

 move by crawling, rarely by swimming (Thetis, (Eolis, 

 Glaucus, &c.) Entoconcha is parasitic ; Montacuta 

 striata lives on Spatangiis purpureus ; many species 

 in cold climates hybernate ; Helix Hauffenii, and 

 zoospeum live in caves, and are eyeless. 



The sub-class is divisible into two orders, Branchiata 

 (Branchiogasteropoda) and Pulmonata. 



i. Branchiata respiration aquatic; larva with a conspi- 

 cuous velum ; intestine with a haemal flexure or straight. 

 This includes the following sub-orders : 



ist. Abranchiata separate gills none; surface ciliated; 



rachiglossate ; larva with a shell which is soon lost. The 



families are, Pontolimacidoe body flat ; foot broad ; tentacles 



filiform or none (Rhodope),f or as two, lateral, head crests 



olimax); anus dorsal. 2. Phyllirhoidaj free swiin- 



flcxurc of the intestine. 4th. The loss of the velum and full 

 lula, &c. 



. : thus 



um, ChcU . &C. 



fl: .irt, nor tentacles. 



...J-likc in Ccnia and JJcrmatobranthia. 



