42 



COMPARATIVE ANATOMY 



CHAP. 



Iii Chaetoderma the foot finally atrophies, and the medio-ventral groove also 



lisappears. 



The long series of undoubtedly primitive characteristics in these two groups the 



Placophora and Solenogastres obliges 

 us to place them, as we shall have 

 repeatedly to point out, near the root 

 of the Molluscan phylum. In some 

 points the Solenogastres are perhaps 

 more primitive than the Pul>/j>/itc<i- 

 /iltum, and the vermiform body, the 

 slight development of the mantle, the 

 foot and the gills have been thought 

 to be primitive characteristics. More 

 recently, however, it has been main- 

 tained, as the present writer thinks, 

 with justice, that these conditions are 

 rather the result of secondary adapta- 

 tion to a limicolous habit of life (most 

 Solenogastres inhabiting mud). The 

 shell, mantle, gills, and foot are such 

 essential characteristics of the Mol- 

 lusca that we must assume their 

 existence in the racial form. 



The series Chiton, Chitoncllus, 

 Neomenia,, Chaetoderma does not, there- 

 fore, illustrate for us the rise and 

 development of typical Molluscan 



characteristics, but rather their progressive obscuration and disappearance. 



Fi<;. >'-'i. Transverse section of Proneomenia 

 Sluiteri in the region of the mid-gut. 1, Mid-^ut ; 

 2, rudimentary Toot ; 3, sepia projecting into the mid- 

 ^ut ; 4, testicnlar portion of the gonad ; 5, ovarial 

 portion of the same ; 6, thick cuticle secreted by the 

 epithelium. 



C. Gastropoda. (Cf. Sketch of Outer Organisation, pp. 30-33.) 



Integument. 



The free edge of the mantle, which takes the chief part in the 

 formation and growth of the shell, is particularly rich in mucous, pig- 

 ment, and calcareous glands. 



The epithelium is ciliated over areas of varying extent, especially 

 in aquatic Gastropods. In many of the shell-less Opisthobranchia the 

 whole surface of the body is ciliated. 



The remarkable marking and colouring of the integument especi- 

 ally seen in the Nudibranchia are caused by pigment cells, which are 

 more often found in the cutis than in the epithelium. 



Where there is no firm shell, calcareous granules or spicules may 

 be found scattered throughout the cutis. 



In several Nudibranchia stinging cells have been discovered in 

 the integument. 



Mantle, Visceral Dome. 



The mantle fold is, as a rule, well developed in Gastropods, and 

 covers a spacious pallia! cavity. Whenever the fold is small or alto- 

 gether Avanting, the condition is secondary rather than primitive. 



