LHE MOLLUSCA 3 
pedigrees of the sub-groups more certainly traced ; and for this 
reason no phylum, with the possible exception of the Echinoderma, 
has, in recent years, yielded such fruitful results to the investigator. 
Il. Tok MorpHoLocy AND Lirre-HIstory OF THE MOLLUSCA. 
1. General Description and External Characters.—The Mollusca are 
Coelomocoela with a distinct coelom and haemocoel. The latter has 
undergone a great development by phleboedesis (see Professor Lan- 
kester’s introduction in Part II. of this treatise), and the coelom there- 
fore is proportionately reduced. In the adult condition there are two 
recognisable coelomic cavities, the pericardial coelom and the true 
gonocoel or gonadial cavity. These two may be in communication 
with one another, but more frequently they are separate. The 
haemocoel is completely closed, and is probably the remnant of the 
embryonic blastocoel. The coelom, on the contrary, communicates 
with the exterior by coelomoducts and coelomopores. The coelomo- 
ducts are part of the primitive protocoelom ; they first served as 
gonaducts, but afterwards were adapted to excretory functions and 
became excretory organs or uroducts. In some cases, however, they 
retain part of their primitive character and serve the double purpose 
of uroducts and gonaducts (Fig. 5°, 9, 7, j). 
The external features of a mollusc, though very variable, nearly 
always admit of a division of the body into three tegumentary 
regions or organs. The most anterior division is the head: it bears 
the mouth, appendages of various kinds, and nearly all the organs 
of special sense. The second division is ventral, and has the form 
of a highly developed tegumentary projection of variable shape : 
this is the foot, the chief organ of locomotion. The third division 
comprises the dorsal part of the body, covered by a calcified cuticle 
or protective shell, the shape of which is subject to great variation 
in the different subdivisions of the phylum. This dorsal region is 
known as the mantle or pallium. 
The free surface of the body is covered by an epithelium, which 
is frequently ciliated. The epithelium is richly supplied with 
glandular cells which secrete an abundant mucus, serving to keep 
the skin moist and supple. In some cases, e.g. Phyllirhoé and 
Pholas, the glandular secretion is phosphorescent. The epithelium 
is also richly supplied with nerve-endings, and some of the epithelial 
cells may secrete chitinous or calcareous spicules, which remain 
embedded in the integument (Amphineura). 
The greater part of the external, or at least the originally 
external, pallial epithelium secretes a shell (Fig. 1, sh). The shell 
consists of a matrix of a chitin-like substance, conchyolin, im- 
pregnated with mineral salts. The latter may be present in such 
abundance as to form 95 per cent of the shell substance. ‘The chief 
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