242 Iiitrodudion to Animal Morphology. 



with no articulated appendages, modified by unequal 

 lateral development, and by a fusion of metameres. 



The integument and muscular layers unite to form 

 a contractile envelope,* consisting of — ist, an epi- 

 dermis, ciliated in the embryo,! becoming superficially 

 tesselated in the adult, or forming a laminated cuticle, 

 pierced by non-glandular pore canals, as in some 

 Cephalopods ; 2nd, a dermis of connective tissue, 

 sometimes cartilaginous in parts (Carinaria, Loli- 

 gopsis), containing glands,^ pigment cells,§ often 

 granules, spicules, or networks of carbonate of lime ;|| 

 sometimes with follicles giving origin to bristles like 

 those of Chaetopods,1[ or rarely with clusters of cnidae 

 like those of Turbellaria ;** 3rd, a muscular layer, 

 never separable from the last, usually consisting of in- 

 terrupted bundles of transverse fibre cells externally, and 

 of a more continuous inner layer of longitudinal fibres. 



The head of the larva is furnished with an expan- 

 sion of the integument for locomotion, the Velum, 



* When there is much connective tissue, as in the clear dermis of 

 Heteropods, the capacity of form-change is small. The development of a 

 shell also limits its contractility. 



t In the adult, ciliation is confined to the gills, except in a few forms, 

 as Phyllirhoe. 



X These are — ist, unicellular glands like those in worms; to these the 

 goblet cells, scattered on the surface of some molluscs, seem to be related ; 

 2nd, mucous glands, with ducts scattered over the surface as csecal follicles ; 

 3rd, colour glands secreting a deep purple fluid, as in Murex, Aplysia, and 

 Purpura. These are usually limited to areas on the surface. 



§ Chromatophores, or connective corpuscles fiUed with pigment, having 

 radiating, muscular fibre cells (?) attached to their periphery, whose contraction 

 causes the cells to assume a stellate appearance, found in Cephalopoda and 

 Pteropoda. 



II As in Doris, &c. 



IT As on the mantle edge of Brachiopods and Chitons. 



«* As in the dorsal papillae of CEolidse. 



