THE MOLLUSCA 5 
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, ¢’), 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 Plewrobranchus 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 
