294 . ERNEST AVAREEN. 



produced by the presence of a parasitic larva of a Pj^cno- 

 goiium (PI. XLVII, fig. 18, g.). 



An egg of the Pycnogonum finds its way into the diges- 

 tive cavity of a hydranth. It is, of course, quite possible that 

 the parent may actually deposit its eggs in the polyp. The 

 polyp contracts, loses its tentacles and the characteristic 

 dilatation of the coelenteron, and becomes in fact a closed sac 

 (fig. 18, chy.) ; i/.f. is the young developing embryo. The 

 polyp-sac then elongates and projects out of the mouth of the 

 hydrotheca (fig. 19, d.g.), and ultimately it grows and produces 

 a large gall (fig. 18, g.) into which the embryo has passed. 



The gall is remarkable for its structure : it is lined by a 

 layer of ectoderm (fig. 20, e.g.) like a hydrotheca, and it 

 generally terminates in four marginal teeth and a well- 

 developed operculum {op.). The length of a gall is about 

 1"50 mm., and its greatest breadth 0"65 mm. Thickness of 

 perisarc 20 ju. The stalk of the gall sometimes shows a 

 tendency to be annulated. 



The outgrowth of the original hydranth has a well-defined 

 coelenteron, lined by a regular endodermal epithelium, outside 

 of which is a thin ectoderm. ' Sometimes the structure is 

 surmounted by a set of well-developed tentacles (figs. 18 and 

 20, te.) ; but a definite mouth has not been observed. At 

 other times the structure ends blindly without tentacles. 



The embryo, lodged in the coelenteron, grows, and there 

 are developed two chelicer^ (fig. 18, ch.) ending in two little 

 plates or lappets, Avhich are perhaps of the nature of claws. 

 In an older embryo two long curved claws may be found in 

 this position (fig. 18, cL). 



In a longitudinal dorso-ventral section of the young 

 embryo (fig. 20) there niay be observed the following 

 characters : 



(1) The embryo surrounded by a cuticle (r.). 



(2) The stomoda3um {8t.) dipping down towards the arch- 

 enteron (Ar.) which is lined by elongated granular endoderm 

 cells. These cells are placed remarkably separate from one 

 another, and are frequently branched {end.). 



