220 ZOOLOGY sect. 



or mid-yolked, but soon the protoplasm accumulates at one end 

 and the yolk at the opposite end of the developing embryo, pro- 

 ducing a tclolecithal 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). 



4. The presence, for the first time in the ascending animal 

 series, of a true middle embryonic layer or mesoderm. In the 

 other Ccelenterata. as well as in the Sponges, two embryonic layers 

 only are formed, and the intermediate layer of the adult is formed 

 by the comparatively late separation of muscle-cells and connec- 

 tive-tissue fibres either from ectoderm or endoderm. In the 

 present case a definite layer of mesoderm cells becomes separated 

 from the endoderm during the gastrula stage. 



2. Distinctive Characters and Classification. 



The Ctenophofa are pelagic Ccelenterata 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 

 meridional rows or swimming-plates. 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 



