36 HTDROZOA. 



phyllia.' These derive their origin from both 

 ectoderm and endoderm, though chiefly from the 

 former, and always enclose a ' phyllocyst/ or csecal 

 process of the somatic cavity. 



The pigment-granules of the Hydrozoa are, in 

 general, of irregular form, and, though usually 

 found in the ectoderm, are by no means, as we 

 have seen, peculiar to it, 



6. Muscular System and Orgasms of l^oco- 

 motion.— In connection with the more or less con- 

 ti^actile body-substance itself, separate muscular 

 fibres, whose arrangement is most frequentl}^ longi- 

 tudinal, present themselves among many Hydrozoa^ 

 and especially in the highly contractile coenosarc of 

 the Calycojphoridce and Physophoridce. Similar 

 fibres may also be traced in the walls of the polypites 

 and tentacula, or on the concave surface of the 

 swimming organs, with which several of the oceanic 

 species are provided. They occur most abundantly 

 in the ectodermal layer (fig. 5, b). In the Medusidce 

 and Lucernaridce both radiating and circular fibres, 

 not without distinct indications of transverse stria- 

 tion, have, by different observers, been detected. 



Special swimming organs, or nectocalyces, are 

 found in the Ccdycophoridw, Medusidce, and many 

 of the Physophoridce (figs, 20, 22, and 23). Each 

 nectocalyx is a cup, or bell-shaped body, the open 

 cavity of which is provided with a muscular lining, 

 and has received the name of * nectosac' Around 

 the margin of the nectosac, the wall of the 

 nectocalyx is produced inwards, forming a shelf- 

 like membrane, or ' veil.' By means of this mem- 

 brane, which is highly contractile, the aperture of 

 the bell may be more or less narrowed. Within 



