EMBRYONIC FISSION IN OYCLOSTOMATOUS POLTZOA. 15 



pides in the zooecia contiguous to the ovicell. It is well 

 known that the thick calcareous ectocyst of the Cyclostomata 

 is perforated by pores. On decalcifying a colony, and stain- 

 ing what is left, it can be easily shown that all the zooecia are 

 in organic connection by means of the funicular tissue, which 

 passes through the pores from one zooecium to another, and 

 from the zooecia to the ovicell. It can hardly be doubted that 

 the nutriment at the expense of which the larvae develop is 

 provided by means of the protoplasmic network which thus 

 connects all the individuals of a colony. The ovum is ex- 

 tremely minute, although it gives rise to a massive primary 

 embryo ; and this to numerous free larvae, each of which is 

 very many times larger than the original ovum. This rapid 

 growth — to say nothing of the development of an extensive 

 reticulum of funicular tissue in the ovicell itself — can only 

 depend on the existence of pores by which the ovicell is con- 

 nected with zooecia which possess functional polypides. 



In fig. 2 the ovum is completely surrounded by the polypide- 

 bud, whose tentacle-sheath has considerably increased in size ; 

 while in fig. 3 further alterations of importance have taken 

 place. The tentacle-sheath has grown very much larger; but, 

 so far as this structure and the invagination which forms the 

 aperture are concerned, the ovicell still resembles an ordinary 

 zooecium. The proximal part of the polypide-bud, which in 

 the younger ovicell was practically indistinguishable from 

 the corresponding structure in an ordinary zooecium, has now 

 become much modified. The egg is now completely sur- 

 rounded by it ; and the polypide-bud has in fact transformed 

 itself into a round mass of cells which may be termed the 

 " follicle." The ovum lies partly surrounded by a cavity in 

 this follicle. 



The fact that the distal endocyst is not in contact with the 

 ectocyst is probably due to shrinkage brought about during 

 decalcification. The side-walls of the zooecium are of course 

 calcified (c/. fig. 19), while the distal ectocyst forms an uncal- 

 cified membrane stretching across the mouth of the funnel 

 formed by the ovicell. 



