396 THE MUSSEL chap. 



formed a glandular pouch, which secretes a bunch of silky 

 threads, the byssus (/). The larva is now called a. glochidium, 

 the subsequent history of which furnishes another example 

 of a means of ensuring dispersal in a sedentary animal (com- 

 pare pp. 272 and 278). 



The glochidia, entangled together by means of their 

 byssal threads, escape from the gills of the parent by the 

 exhalant siphon, and eventually attach themselves by their 

 hooked valves to the body of a passing fish, such as a 

 stickleback. Here they live for a time as external parasites, 

 gradually undergoing metamorphosis ; and finally drop from 

 the host and assume the sedentary habits of the adult. 



This mode of development is exceptional amongst bivalves, in most 

 of which (e.g. oyster) the egg gives rise to a larva resembling the 

 trochosphere of many annelids (p. 340), the prostomial' region then 

 growing out into a thickened rim which bears the circlet of cilia in front 

 of the mouth, the larva at this stage being distinguished as a veliger. 



The Mussel belongs to the phylum MoUnsca, which 

 includes, in addition to the bivalved " shell-fish "• — such as 

 mussels, oysters, cockles, — the snails, slugs, whelks, peri- 

 winkles, &c. — most of which possess a univalved shell, as 

 well as the cuttle-fishes and their allies. These are all 

 sharply distinguished from the Arthropods by the absence 

 of segmentation, and by having, as a rule, an exoskeleton in 

 the form of a shell. The bivalves are included in the class 

 Pekcypoda or Lamellibranchiata, the essential structure of 

 which you will have learnt from your study of the Mussel. 



You have now examined examples of the following chief divisions 

 or phyla of the animal kingdom (compare p. 219) : 



Protozoa. Anuulata. Mollusca. 



Coelenterata. Arthropoda. Vertebrata. 



In addition to these, you will probably have seen, when dissecting the 

 frog, certain parasites. One of these, Polystomitm (p. 33), belongs 



