ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 721 



glands are degenerate or absent may be due to excretion inside tbe 

 auricles (Pecten, Spondylus, Ostrea, Lima, Pinna, Meleagrina), or to 

 concretions in the mantle (Area). 



The double character of the heart-chamber in Area is a secondary 

 result of the marked development of the posterior retractor. 



The ciliated funnel of the kidney is not absent in Pecten or Spondylus, 

 and lies in front of, and dorsal to the atria. 



The union of the two atria in Monomyaria in front of the ventricle 

 is the same as the posterior union in Area, Pectunculus, Mytilus, and 

 Lithodomus, the change in position being due to the torsion of the body. 



The position of the heart behind the posterior adductor in Teredo is 

 due to the posterior and ventral displacement of the body. The single 

 aorta is due to the union of the anterior and posterior. The anterior 

 adductor is present, as in all other Pholadidte, but is weakly developed. 



MoUuscoida. 

 /3. Bryozoa. 



Embryogeny of Ectoproctous Bryozoa.*— Mr. S. F. Harmer has 

 studied, at Koscoff, the development of Alcyonidium polyoum. The ova 

 are large and contain a number of vitelline spherules, which, in the 

 early stages of development, are found indiiferently in all the cells. 

 The segmentation is of the remarkable type which appears to be 

 characteristic of the Ctenostomata and Cheilostomata. At the 48-stage 

 the aboral region has two longitudinal rows of four cells, which are 

 disposed symmetrically right and left of the median plane, and occupy 

 the centre of the aboral surface ; there is a complete circle of eight cells 

 which surround the central group, and are themselves surrounded by a 

 peripheral ring of sixteen cells, which are, as Barrois has shown, the 

 commencement of the ciliated circlet. The oral half has a central group 

 of four large cells, which are surrounded by twelve peripheral cells. 

 The segmentation-cavity is, at this stage, relatively large, but is partly 

 filled by four cells which are placed immediately above the central oral 

 cells, from which they are probably derived ; these four cells are the 

 commencement of the hypoblast. At a slightly more advanced stage the 

 blastopore appears as a well-marked depression, which is continuous 

 with a rather irregular cavity surrounded by several large hypoblastic 

 cells. The segmentation-cavity becomes completely obliterated by the 

 internal cellular mass, and the various organs of the larva begin to make 

 their appearance. 



The alimentary canal of the embryo is well developed ; it consists of 

 a vast stomach, bounded by an extremely irregular epithelium; the 

 oesophagus, which is perhaps formed as a stomodceum, has a very 

 narrow cavity ; the mouth is larger and more evident in early than in 

 later stages. There is some reason for thinking that the region immedi- 

 ately behind the opening of the sucker (which is placed a little behind 

 the middle of the ventral surface) represents the anal region. If this be 

 really the case, the embryo is entoproctous. When the alimentary canal 

 has acquired its maximum of development, which it does at an early 

 stage, the cavity of the stomach may be justly called gigantic. It is 

 not, however, easy to make out the epithelium which lines it, for it is 

 composed of a mass of vitelline spherules enveloped in protoplasm with 



* Arch. Zool. Exper. et Gen., v. (1887) pp. 413-58 (2 pis.). 



