LESSON 44.] NUTRITION IN THE TUNICATE MOLLUSCA. 155 



LESSON XLIV. 



OEGANS OF NUTRITION IN THE TUNICATE MOLLUSCA. 



683. The Molluscous, or soft-bodied animals, appear to be organ- 

 ized for the perfecting of the nutritive function, to the development 

 of which every other consideration gives way. 



684. The lower orders of this class (Tunicata, Brachiopoda, 

 Lamellibranchiata) are headless, and called, therefore, Acephala 

 (a, without ; cephale, a head) ; those with a head are the Pteropoda 

 (wing-foot), Gasteropoda (belly-foot), and Cephalopoda (head-foot). 



685. The Tunicata are so called because their bodies are en- 

 closed in a gelatinous membrane, or sac ; they breathe either by a 

 vascular membrane, or by a ribbon-shaped gill stretched across the 

 common visceral cavity. There is no shell or calcareous deposit in 

 these animals ; moreover, in common with the headless Molluscs, 

 they are fixed to the rocks, and other submarine substances. 



686. The Brachiopods (brachion, the arm ; pous, the foot) pos- 

 sess a bivalve shell, have two long, spiral, strongly ciliated arms, de- 

 veloped from the sides of the mouth, and breathe by means of their 

 vascular mantle. 



687. The Lamellibranchiata are bivalve Molluscs, breathing by 

 means of lamellated gills (hence their name) attached to the mantle. 

 The Oyster and the Mussel are the best examples. 



688. The Pteropods swim by two wing-like muscular appendages 

 attached to the sides of the (supposed) head. 



689. The Gasteropods are the slugs and snails ; they walk upon 

 a foot, of greater or less dimensions, attached to the under surface 

 of the ventral surface hence their name, belly-foot. 



690. The Gephalopods have their locomotive organs attached to 

 the head, generally in the form 252 



of muscular arms, or tenta- 

 cles ; this is the only order 

 of the mollusca possessing an 

 internal skeletpn. 



691. The Tunicate) are 

 frequently found in groups, 

 forming compound animals, 

 like many of the Polypes. 

 The outer gelatinous tunic is 



