218 



ZOOLOGY 



SECT. 



embryos goes through a process similar to segmentation. Of the cells thus 

 formed one gives rise to the axial cell ; the others, increasing in numbers and 

 becoming smaller, gradually grow over the axial cell until they at length 

 completely enclose it. This process is a simple form of epiboly, and the part 

 which is last to be covered by ectoderm cells (the posterior 

 end) corresponds to the blastopore. The embryo increases 

 greatly in length, and escapes from the interior of the parent 

 by perforating the body-wall. 



The germ destined to give rise to an infusoriform embryo 

 likewise undergoes segmentation and epibolic gastrulation. 

 The fully formed infusoriform embryo (Fig. 164) is pear- 

 shaped, the broader (head) end being that which is directed 

 forward in swimming, and is completely bilaterally symmetri- 

 cal. The ectoderm cells enclose an axial cell containing several 

 smaller multi-nucleated cells which can be ejected apparently 

 voluntarily. The fate of these embryos is not known ; it has 

 been suggested that they may be males, and that the cells 

 thrown out may be male cells. 



The Heteroct/emiiln; which are also parasites of the Cephalopoda, resemble the 

 Dicyemidse in most respects, but want the head. 



The family Orthonectidce comprises only one genus Rhopalura with two 



FIG. 104. Embryo 

 of Dicyema 

 paradoxum. 



(From Bromi's 

 Thierreicli.) 



FIG. K.,. Rhopalura Giardii, 



(From Bruim's Thicmich.) 



FIG. 100. Rhopalura Giardii, 



(From Broun's Tliierrciclt.) 



species, one of which is parasitic in the genital pouches of an Ophiuroid, while 

 the other is a parasite of a Nemeitean worm. In form they are spindle-shaped, 



