iv PHYLUM CCELENTERATA 217 



their way on to the free surface of the cells, and become supported 

 on four groups of fused cilia. Four outer groups of cilia unite with 

 one another to form the bell (sk.). 



The most noteworthy points in this somewhat complex process 

 of development are the following : 



1. The distinction between a purely protoplasmic part of the egg 

 and a yolk-containing portion. In the Hydrozoa and Actinozoa 

 the yolk-material is small in amount and evenly distributed, the egg 

 being described as alecithal or microlecithal. In the present instance 

 the yolk is at first accumulated in the centre of the egg, which is 

 thus centrolecithal or mid-yolked, but soon the protoplasm accumu- 

 lates at one end and the yolk at the opposite end of the developing 

 embryo, producing a telolecithal or end-yolked condition. 



2. The fact that segmentation is unequal, there being a distinc- 

 tion into large cells or megameres, containing yolk, and purely 

 protoplasmic small cells or micromeres. 



3. The formation of a peculiar type of gastrula by epiboly or 

 overgrowth, the ectoderm cells (micromeres) growing over and 

 partly enclosing the endoderm cells (megameres). 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Ctenophora are pelagic Crelenterata in which the formation 

 of colonies is entirely unknown. No indication of a polype-stage, 

 so characteristic of the remaining Ccelenterata, can be detected 

 either in the adult or in the embryonic condition. Ciliary move- 

 ment, instead of being a merely embryonic form of locomotion as 

 in the preceding classes, is retained throughout life, the cilia being 

 fused to form comb-like structures, which are arranged in eight me- 

 ridional rows or swimming-plates (costae). Tentacles, when present, 

 are usually two in number, situated in opposite (right and left) 

 per-radii, and retractile into pouches. The enteron communicates 

 with the exterior by a large stomodseum which functions as the 

 chief digestive cavity. From the enteron is given off a system of 

 canals, the ultimate branches of which are adradial and have a 

 meridional position, lying beneath the swimming-plates ; a single 

 axial canal is continued to the aboral pole, where it commonly 

 opens by two excretory pores. There are no gastric filaments. 

 The central nervous system is represented by a ciliated area on 

 the aboral pole, and is connected with a single sensory organ, 

 having the character of a peculiarly modified lithocyst. The 

 gonads of both sexes are lodged in the same individual, the ovaries 

 and testes being formed on opposite sides of the meridional canals. 

 The oosperm undergoes unequal segmentation, the gastrula is 

 formed by epiboly or overgrowth, and a definite mesoderm is 

 established during the gastrula stage. There is no alternation of 

 generations ; but in some cases development is accompanied by a 

 well-marked metamorphosis. 



