CNIDARIA 77 



to the tj-pe of superGciul cleavage, which is wide-spread 

 among the Arthropoda. 



In general the cleavage stages of the Alcyonaria are characterized by 

 the absence of the cleavage cavity. Monoxenia forms an exception. 

 Here, according to Haeckel (No. 78), there are produced in the course of a 

 very regular cleavage a typical ctEloblastula-stage and a gastrula in- 

 vaginata. 



In the nioruhx a difference can early be recognized (Fig. 

 32 A) between a superficial cell-layer (ectoderm) and an 

 inner cell-mass (entoderm). This diffei-ence becomes more 

 marked in later stages (Fig. 32 JB, C). The ectoderm cells 

 by progressive division are metamorphosed into prismatic 

 elements, which constitute a columnar epithelium (Fig. 32 

 C). Those of the inner cells lying next to the ectoderm also 

 arrange themselves (Fig. 32 (7, en) into an epithelial layer 

 (the permanent entoderm), whereas the elements lying at 

 the centre undergo a process of degeneration. The cell 

 boundaries here become indistinct ; vacuolar spaces make 

 their- appearance, and soon coalesce into a common internal 

 cavity (the beginning of the gasti-al cavit}', h) ; finally, this 

 entire cell-mass is metamorphosed by fatty degeneration into 

 a kind of detritus (il), which is gradually resorbed. At the 

 same time a fine, structureless, hyaline membrane (the sus- 

 tentative lamella) is secreted between the ectoderm and 

 the permanent entoderm. 



While these internal changes are taking place, the body 

 elongates and gradually assumes an ovoid or, with increasing 

 length, a vermiform shape, and its surface becomes covered 

 with close-set cilia; thus the swarming plamda-staye is 

 developed (Fig. 32 D). The planula exhibits a somewhat 

 broadened (aboral) end, which is directed forwards during 

 motion, and a posterior (oral), more pointed pole. At the 

 expiration of the swarming stage the larva attaches itself 

 by means of its broadened antei-ior end to some convenient 

 support. By a gradual shortening in the direction of the 

 longitudinal axis the larva passes from the elongated into 

 a low placentiform shape (Fig. 33). 



At about the same time with the attachment, the in- 



