CCELENTERATA : HYDROZOA. loq 



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veloped near their basal or proximal ends, and provided with 

 lateral branches ending in saccular cavities," and furnished 

 with numerous thread-cells. The proximal ends of the poly- 

 pites usually bear certain overlapping plates, of a protective 

 nature, which are termed " hydrophyllia " or " bracts." They 

 are composed of processes of both ectoderm and endoderm 

 (fig. 27, 3 d\ and they always contain a diverticulum from the 

 somatic cavity, which is called a " phyllocyst." The Calyco- 

 phoridcR always possess swimming-bells, or " nectocalyces," by 

 the contractions of which the hydrosoma is propelled through 

 the water (fig. 27, 2). The nectocalyx in structure is very 

 similar to the " gonocalyx " of a medusiform gonophore, as 

 already described ; but the former is devoid of the gastric or 

 genital sac the "manubrium" possessed by the latter. Each 

 nectocalyx consists of a bell-shaped cup, attached by its base 

 to the hydrosoma, and provided with a muscular lining in the 

 interior of its cavity, or "nectosac." There is also always a 

 "velum" or " veil," in the form of a membrane attached to 

 the mouth of the nectosac round its entire margin, and leaving 

 a central aperture. The peduncle by which the nectocalyx is 

 attached to the hydrosoma conveys a canal from the somatic 

 cavity, which dilates into a ciliated chamber, and gives off at 

 least four radiating canals, which proceed to the circumference 

 of the bell, where they are united by a circular vessel ; the 

 entire system constituting what is known as the system of the 

 " nectocalycine canals." In the typical Calycophorida two 

 nectocalyces only are present, but in some genera there are 

 more. In Praya the two nectocalyces are so apposed to one 

 another that a sort of canal is formed by the union of two 

 grooves, one of which exists on the side of each nectocalyx. 

 This chamber, which is present in a more or less complete 

 form in all the genera, is termed the " hydrcecium," and the 

 ccenosarc can be retracted within it for protection. 



The reproductive bodies in the Calycophorida are in the 

 form of medusiform gonophores, which are budded from the 

 peduncles of the polypites, becoming, in many instances, de- 

 tached to lead an independent existence. In some Calycopho- 

 rida, as in Abyla, " each segment of the ccenosarc, provided 

 with a polypite, its tentacle, reproductive organ, and hydro- 

 phyllium, as it acquires a certain size, becomes detached, and 

 leads an independent life the calyx of its reproductive organ 

 serving it as a propulsive apparatus. In this condition it may 

 acquire two or three times the dimensions it had when at- 

 tached, and some of its parts may become wonderfully altered 

 in form" (Huxley). To these detached reproductive por- 



