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 ; in Marseniathe larval shell (Echino- 

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

 some slugs there is a pulsating caudal sac (in Limnaea 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 exists, and often persists in 

 the adult, as in Limnaea ; 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 

 are 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, CEolis, 

 Glaucus, &:c.) Entoconcha is parasitic ; Montacuta 

 striata lives on Spatangus 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 : — 



I St. Abranchiata — separate gills none; surface ciliated; 

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

 families are, Pontolimacidae — body flat ; foot broad; tentacles 

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

 (Pontolimax) ; anus dorsal. 2. Phyllirhoidae — free swim- 

 flexure of the intestine. 4tli. The loss of the velum and full development 

 of the mantle, radula, &c. 



* Larvos of this class have been described as distinct genera : thus 

 ;Macgillivraya is the larva of Dolium, Cheletropis of Murex, &c. 



t This, the lowest Gasteropod, has no eyes, mantle, heart, nor tentacles. 

 The tentacles are thread-like in Cenia and Dermatobranchia. 



