124 



CQELENTERATA. 



medusae or modified medusae by budding; and medusae which 

 arise directly from the egg. The polyps are generally attached 

 permanently to foreign bodies, and are of small size; but in the 

 Siplwnopliora the whole colony is free-swimming. The medusae 

 are also of small size and possess a velum. When skeletal structures 

 are present they consist, as a rule, of a more or less horny secretion 

 of the ectoderm (j>erisark}. In one order, however, the secretion 

 is calcareous (Hijdrocorattinae). 



The colonies very often present, in a well-marked manner, the 

 phenomenon of polymorphism, and in describing the various 



- en/ 



ch 



FIG. 109. Development of hydroid polyps (from Chun, I and II 

 after Metsclmikoff, III after Allnian). I. Young larva of (.'bjtla 

 flat'idiila six hours after attachment, ek ectoderm ; en radially 

 pouched endoderm (rudiment of hydrorhiza); g enteric space; 

 liy cylindrical projection which constitutes the hydrocaulus. 

 II. Larva of Clytia one day after fixation; the hydrorhiza forms 

 a chambered disc from which the cylindrical hydrocaulus pro- 

 jects ; nematocysts are present in the ectoderm. III. Older 

 larva of Eudendri-um mmosum ; the hydrocaulus invested by the 

 chitinous perisark (o/i) projects from the discoidal hydrorhiza; a hydrocephalis with a 

 tentacular circlet (t) has been developed at the free end of the hydrocaulus ; the mouth 

 is not yet formed. 



modifications which may be present it will be convenient to explain 

 some of the more common terms which are iised in the cumbrous 

 and complicated nomenclature of the group. 



The fertilized ovum very generally gives rise to an oviform free-swimming 

 larva the planula (Dalyell*), consisting of an outer layer of ciliated .ectoderm 

 and an inner hollow mass of endoderm. After a short time it loses its cilia and 

 secretes a thin cuticular covering the perisark and becomes attached by one 

 end (Fig. 109). The free end elongates and develops a terminal mouth and 



1847. 



Sir J. Graham Dalyell, Rare and Remarkable Animals of Scotland, London, 



