DEVELOPMENT OF FLUSTEELLA HISPIDA. 465 



onwards the development of the larval organs, with the 

 exception of the stomach, has been already so fully described 

 by Prouho (20) as to make detailed description unnecessary. 



The internal sac has become much elongated, so that it now 

 occupies the greater part of the interior of the larva. Its 

 lining epithelium has become much thickened, and has already 

 lost all trace of cell structure. 



The pharynx has altered somewhat in appearance, owing 

 to the loss of the large globules already described. 



The pyriform organ is now fully developed. On the 

 exterior two depressions are noted — an anterior depression 

 (Prouho's " fossette supei'ieure '') and a posterior one 

 (Prouho's ''fente ciliee"), and between these lies a tuft 

 of cilia, the " papille du plumet vibratile." Internally, 

 corresponding to the " fossette superieure," is the " sys- 

 teme glandulaire superieure,^' which consists of a single 

 mass of cells lying in the longitudinal axis of the larva, while, 

 similarly corresponding to the " fente ciliee," are two masses 

 of cells, which are placed in the transverse axis of the larva, 

 one on either side of the posterior part of the " systeme 

 glandulaire superieure," and which represent the " systeme 

 glandulaire inferieure." 



The aboral organ (PI. 25, fig. 65 a) at this stage is fully 

 developed. It consists of a tuft of long cilia arising from the 

 thickened patch of aboral ectoderm which lias already been 

 mentioned. This organ is best seen in the living larva, in 

 which it is visible protruding between the two valves of the 

 shell ; and connecting the aboral organ and the pyriform 

 organ is seen the neuromuscular cord. In the living larva 

 the jerking movements of the neuromuscular cord are dis- 

 tinctly visible, but it has not been possible to draw any 

 conclusions from these movements as to the functions of 

 either the pyriform or the aboral organs. The structure of 

 the aboral organ and neuromuscular cord have already been 

 fully described by Prouho (20). As stated by this author 

 the nerve-muscle tract, on reaching the pyriform organ, 

 breaks^into three strands, one of which passes to that organ 



