DEVELOPMENT OF THE PECTINIBRANCHIATA. 339 



After the edges of the mantle have developed themselves into 

 a covering over the back of the animal, a cavity clothed with 

 fine cilia is formed, in which the heart and branchiae lie. The 

 first traces of branchiae which we observed were two indistinctly 

 marked bands arising from the edge of the mantle, and diverging 

 at intervals to unite again and so form loops. We have found, 

 at a more advanced stage, that these bands are tubes taking a 

 very sinuous course, whence they have a certain resemblance to 

 a corkscrew. These sinuosities were better marked above and be- 

 low than in the middle, where they were wider and more closely 

 appressed; an active ciliary motion was distinguishable upon their 

 internal edge (11 p). Loven has shown, in his excellent memoir 

 (op. cit.), that there is an extreme .resemblance, in respect of 

 development, between the Gasteropoda and Acephala ; he has 

 described the mode of formation of the branchiae in the latter, 

 and we have observed even that the production of these organs 

 takes place in a similar manner in Buccinum undatum and Pur- 

 pura lapillus. 



About the period of the formation of the branchiae we ob- 

 served, below, at their union, a vesicle which is formed and de- 

 veloped like the heart. It is ovoid or slightly pyriform, ends 

 below in a very long tube which follows the course of the intes- 

 tinal canal, and eventually, like the latter, becomes lost in the 

 obscure mass of the yelk (11, 12 q). Its walls are thin, semi- 

 transparent, and provided with a multitude of varicose muscular 

 tubes which take a direction both longitudinal and transverse to 

 the vesicle. These tubes have smaller dimensions than those of 

 the heart, and for that reason it is necessary, in order to distin- 

 guish them readily with the microscope, to employ a much 

 higher power than that required to examine the muscles of the 

 heart. 



The contractions of the bladder are strong, and take place 

 from above downwards, whilst the heart contracts from one side 

 to the other. 



When this vesicle dilates it becomes filled with a clear liquid 

 in which a great number of obscure molecules are distinguish- 

 able. We can regard this organ as nothing but the kidneys. 



The period for the development of new organs has now gone 

 by, but all those already formed, become gradually perfected. 



22* 



