vm DEVELOPMENT 411 



between the lobes of the mantle, is formed a glandular 

 pouch, which secretes a long thread, 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 (compare 

 pp. 274, 283, and 320). 



The glochidia, entangled together by means of their 

 byssal threads, escape from the gills of tbe parent by 

 the exhalant siphon, and eventually attach themselves 

 by their hooked valves to the body of a passing fish, 

 such as a stickleback, becoming enclosed by an over- 

 growth of the skin of the fish. Here they live for a 

 time as external parasites, gradually undergoing meta- 

 morphosis, and finally dropping from the host and 

 assuming 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 

 (Fig. 89), the prostomial region then growing out into a 

 thickened rim or velum 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 Mollusca, which 

 includes, in addition to the bivalved " shell-fish " (such 

 as mussels, oysters, and cockles), the snails, slugs, 

 whelks, periwinkles, &c. (most of which possess a uni- 

 valved 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 Pelecypoda 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) : 



1. Protozoa. 3. Annulata. 5. Mollusca. 



2. Crelenterata. 4. Arthropoda. 6. Vertebrata. 



