THE MOLLUSC A 



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. 



II. THE MORPHOLOGY AND LIFE-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 phlebcedesis (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 big , y, i, 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 Avhich secrete an abundant mucus, serving to keep 

 the skin moist and supple. In some cases, e.g. Phyllirlioe 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 



