INTERNAL SKELETON OF MOLLUSCA. 



341 



In this group the mechanism of respiration is combined with the 

 locomotion of the animal. Each time that the muscles of the edge 

 of the mantle relax, water passes into the branchial cavity by its 

 orifice, that is, at each side of the funnel; after it Las bathed 

 the gills it is driven out again by the contractions of the mantle. 

 At this moment the cleft of the branchial cavity is closed, so that 

 the water cannot get out except by the funnel, and this serves not 

 only as the passage by which the water reaches the exterior, but 

 also takes an active share in driving it out. 



Internal Skeleton. 

 § 264. 



In most Mollusca the absence of an internal skeleton is compen- 

 sated by the shells and tests described in § 258; for these serve 

 as supports for the 

 internal parts. 



Independent in- 

 ternal organs of sup- 

 port are, however, 

 found in the Gastro- 

 poda. Two, or some- 

 times four, small 

 plates of cartilage are 

 found in the head of 

 these animals ; they 

 are surrounded by the 

 muscles of the pha- 

 rynx, and arc more or 

 less closely connected 

 with one another. 

 They form the sup- 

 porting apparatus of 

 the radula and the 

 parts connected with 

 it, and also afford 

 points to which some 

 of the pharyngeal 

 muscles, and especi- 

 ally those of the ra- 

 dula, are inserted. 



Cartilaginous or- 



Fig. 179. Section through the head of Sepia offici- 

 nalis. K K' Cephalic cartilages. G Cerebrum, go Gan- 

 glion of the optic nerve, w White body. I Lens, ci Ciliary 

 body, e Cornea, p Eyelid. P Buccal mass, m External, 

 n Internal labial membrane. e/Jaws. r Radula. oe (Eso- 

 phagus, t Arms. 



gans of support are 

 much more highly de- 

 veloped in the Cepha- 

 lopoda. The most important one lies in the head, where it serves 

 as an investment of the nerve-centres, a support for the optic 



