CCELENTERATA. 



143 



Semper 1 . It has an elongated form and is provided with a longitudinal 

 ridge of cilia. There is a mouth at one end of the body and an anus at 

 the opposite extremity. The mouth leads into an oesophagus, which opens 

 freely into a stomach with six mesenteries. In the skin are numerous 

 thread-cells. A mesotrochal worm-like larva, also provided with thread- 

 cells, and ftmnd at the same time, was conjectured by Semper to be 

 a younger form of this larva. 



Ctenophora. The ovum of the Ctenophora is formed of an outer 

 granular protoplasmic layer and an inner spongy mass with fatty 

 spherules. It is enveloped in a delicate vesicle, the diameter of 

 which is very much greater than that of the contained ovum. This 

 vesicle appears to be filled with sea-water, in which the ovum floats. 



Fertilized ova may usually be easily obtained by keeping the 

 captured adults in water from 12 24 hours. The two main 

 authorities on the development of these forms (Kowalevsky, No. 147 

 and 178 and Agassiz, No. 172) are unfortunately at variance on one or 

 two of the most fundamental points. It seems however that the 

 embryonic layers are formed by a kind of epibolic gastrula ; while 

 the true gastric cavity, as distinct from the gastrovascular, is formed 

 by an invagination, and deserves therefore to be regarded as a 

 form of stomoda3um. 



The early stages are very closely similar in all the types so far 

 observed. Segmentation commences by the outer layer of the 

 ovum, which throughout behaves as the active layer, forming a 

 protuberance at one pole, which may be called the formative pole. 

 Close below this protuberance is placed the nucleus. In the median 

 line of the protuberance a furrow appears (fig. 82 A), which 



FIG. 82. FIVE STAGES IN THE DEVELOPMENT OF IDYIA ROSEOLA. (After Agassiz. ) 

 The protoplasmic layer of the ovum is represented in black. 



gradually deepens till it divides the ovum into two. The granu- 

 lar layer follows the furrow so that each of the fresh segments, 

 like the original ovum, is completely invested by a layer of granular 

 protoplasm. Each segment contains a nucleus. A second similar 

 division at right angles to the first gives rise to four segments 

 (fig. 82 B), and the segments so formed become again divided 

 into eight (fig. 82 C). In the division into eight, which takes 

 place in a vertical plane, the segments formed are of unequal size, 



1 "Ueb. einige tropische Larven-formen." Zeit.f. wiss. Zool., vol. xvn. 1867. 



