THE MOLLUSC A 



genera with more or less similar shells might differ considerably 

 from one another in internal organisation, and gradually the 

 structure of the animal, rather than the form of its shell, came to 

 be recognised as the guide to its systematic position. But to this- 

 day there are many genera of Gastropods and Lamellibranchia 

 whose anatomy is unknown, and their classification, founded on the 

 characters of their shells alone, is, of course, provisional. 



The head and foot are fastened to the shell by muscular 

 bundles, which are paired and symmetrical in the Polyplacophora, 

 Scaphopoda (Fig. 181, e'), and Lamellibranchia (retractors of the 

 foot), and in the Cephalopoda (retractors of the head and funnel),, 

 but in the Gastropoda there is a single asymmetrical so-called 

 "columellar" muscle (Fig. 45, co). The fibres of these muscles are 

 attached to the epithelium under the shell. The connective tissue 

 layer which lies beneath the tegumentary epithelium is mesodermic 

 in origin, and is extensively developed in the Mollusca. It invades- 

 the greater part of the original blastocoel and presents the following 

 varieties : (1) plasmatic or vesicular cells, which sometimes give rise 

 to endoplastic calcareous concretions or even to true sub-epithelial 

 spicules, as in Pleurobranchus and various Nudibranchia ; (2) stellate 

 cells ; (3) fibrillar cells. Blood spaces, whose distension causes- 

 turgescence of various parts of the body, are frequently found in 

 this connective tissue. On the other hand, the connective tissue 

 may become compact and form supporting structures ; such are the 

 " skeleton " of the gill filaments, the sub-epithelial " shell " of the 

 Cymbuliidae, and the cartilaginous pieces in the Cephalopods and 

 in the buccal mass of all classes except Lamellibranchs. 



Below, or imbedded in the subcutaneous connective tissue, are 

 muscle fibres forming layers of rectilinear or annular bundles ; but 

 as a rule only a few definite muscular masses can be distinguished. 

 The muscle fibres are of the smooth variety, though in certain cases 

 they appear to be striated, especially in muscles which contract 

 rapidly. Such apparently striated fibres are found in the buccal 

 mass of various Gastropods, in the heart, in the fins of Pteropods 

 and Heteropods, in the siphon of Cephalopods, in the columellar 

 muscle of the larvae of certain Nudibranchs, in the branchial 

 septum (Cuspidaria), in the adductor muscles of various Lamelli- 

 branchs, and especially in the Pectinidae, in which the striation 

 looks almost identical with that of Arthropods and Vertebrates. 

 These fibres, however, are usually regarded as unstriated : they are 

 wholly bi-refringent. The fibrillae are parallel to one another, 

 but in the contracted state are thrown into spirals, which is said 

 to produce the appearance of transverse striation. The subject 

 calls for renewed investigation. 



The tegumentary layer, composed as described of epithelium, 

 connective tissue, and muscles, may attain to a very considerable 



