14 Gt. 0. Sårs. 



discerned, and small bristles are already seen growing forth 

 from the tip and inner edge of the endopodites. The coxal 

 . lobes are likewise well defined, and easily observed, when 

 the larva is viewed from the ventral face (fig. 5), all of 

 them curving inwards. Still , however, the legs are quite 

 immovable and densely crowded, all pointing behind. The 

 length of the larva has now increased to 0,65 mm. 



This is the last larval stage, and the young animal is 

 now about to cast its skin, before entering into the next 

 phase of its development, the postlarval one. 



H Postlarval stages. 



The postlarval development is of very long duration, 

 as compared with the larval one, and indeed rather more 

 than a month would seem to be needed for its full accomplish- 

 ment. It comprises numerous succeeding stages, each being 

 marked off by an exuviation of the animal, whereby, how- 

 ever, only the inner coating of the shell is cast off, the valves 

 being kept in their place, and only supplied with a newly- 

 formed peripheric area. In this manner the characteristic, 

 concentrical lines of growth are formed, the number of which 

 indeed indicates the number of successive exuviations, which 

 have taken place. Among the most prominent features 

 distinguishing this phace of the development from the larval 

 one, may be mentioned, besides the transformation of the 

 mantle-like carapace of the larva into a true bivalve shell, 

 the complete loss of both the masticatory processes of the 

 antennae and of the mandibular feet, and finally, the functional 

 evolution of the branchial legs. 



