METAZOA COELENTERA. 93 



HYDROPHORA. HYDROCORALLINAE. 



The position of the Hydrocorallinae in a natural classi- 

 fication has not been determined with certainty, l but they 

 are placed here provisionally. 



Millepora is a colonial form which secretes a calcareous 

 skeleton (No. 108). The zoons occur in groups (PI. 109, 

 fig. i), each group consisting of a short central zoon and 

 six or eight long ones about it ; in fig. i one of the latter 

 is omitted for the sake of clearness. The central zoon, 

 called the gastrozooid, possesses a mouth and four or 

 more tentacles, while the surrounding dactylozooids are 

 mouthless. The body cavities of these zoons are not di- 

 vided by partitions, but are continuous into the canals 

 which traverse the surrounding flesh or coenosarc in every 

 direction. 



The dactylozooids apparently catch the food and carry 

 it to the gastrozooid, and are therefore tentacular in func- 

 tion while they bear numerous small tentacles on their 

 sides. One of these is represented in fig. 2, much enlarged. 

 Figs. 3 and 4 represent a nematocyst taken from the ten- 

 tacles; these are like those of most hydroids. Fig. 3 repre- 

 sents the thread within the cell and fig. 4 shows it thrown 

 out. Besides this kind of thread cell there is another in 

 Millepora found near the bases of the zoons and shown 

 in figs. 5 and 6. Fig. 7 is a cross section of a gastrozooid 

 showing on the outside the ectoderm nematocysts in dif- 

 ferent stages of development. Inside are the large trans- 

 parent cells which are called gastric because they occur 

 only in the gastrozooid and therefore may be digestive in 

 function. The muscles by which the zoons contract are 

 shown in fig. 8 which is a diagram of the longitudinal 



1 For a discussion of the different views on the subject, see Moseley, 

 Chall. Rep., Zoo!., II, part 7, 1881, p. 98; also Hickson, Quart. 

 Journ. Micr. Sci., XXXII, 1891^.375; Proc. Zool. Soc. London, 

 1898, p. 246. 



