242 Introduction 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,f 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. 



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

 as Phyllirhoe. 



These are 1st, 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 caecal 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 filled 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. 



|| As in Doris, &c. 



II As on the mantle edge of Brachiopods and Chitons. 



* * As in the dorsal papillae of CEolidas. 



