HYDROID ZOOPHYTES. 9 



consisting of a dense mass of branching and anastomosing tubes (fig. 6). Each 

 tube is covered with its own very thin sheath of perisarc (fig. 33) and there is no 

 common sheath of coenosarc covering the hydrorhiza as a whole, such as we find 

 in the, Podocorynidae. In the central parts of the hydrorhiza the tubes are closely 

 packed, anastomose freely, and are disposed in several layers. At the periphery 

 however the tubes are reduced to a single layer, become more dispersed, and 

 anastomose slightly (fig. 3). There are three kinds of zooids. 



In the central part of the colony there are gastrozooids and blastostyles (gonozooids) 

 (fig. 6) ; at the periphery, gastrozooids and dactylozooids (fig. 5). The gastrozooids 

 (figs. 5 and 6, gz.) are 1-2 mm. in . height. They exhibit a conical hypostome 

 surrounded by a circlet of six to ten simple tentacles each about • 5 mm. in length. 



The dactylozooids (fig. 5, d.) are short finger-like structures 0*25 mm. long by 

 0"06 in diameter, terminating in a pad distally which bears a battery of nematocysts. 

 The dactylozooids appear to be covered with a chitinous perisarc, but in sections 

 it is seen to be extremely thin or absent at the distal extremity. We have some 

 specimens with the nematocysts discharged, and there can be little doubt therefore 

 that the battery is functional. These dactylozooids have no fentacles. The 

 blastostyles are shorter than the gastrozooids, and usually exhibit neither mouth 

 nor tentacles (fig. 6, hi.). There is always a short conical hypostome armed with 

 nematocysts, and occasionally this is surrounded by a circle of four rudimentary 

 tentacles (fig. 6, bl. t.). 



The body of the blastostyle is usually considerably dilated and has a 

 superficial resemblance to a simple ovoid sporosac. 



Gonosome. — The medusoid structure of the gonosome is completely reduced 

 in the male, but as no female blastostyles were found we can make no statement 

 to the efi'ect that the same is true of both sexes. The sperm cells (fig. 33, sp.) are 

 found in a dense cluster between the ectoderm and endoderm even in the youngest 

 blastostyles we have examined (0 ' 1 mm. in length), and neither in these nor in the 

 older blastostyles can we find any true medusoid structures. 



FAMILY PODOCOEYNIDAE. 



In this family we include Podoeoryne (Sars), Hydractinia (Allman), and other 

 less well-known allied genera. The very interesting new genus Hydrodendriwn 

 (Nutting) may, we consider, be now included in this family, as the species Hydractinia 

 dendritica connects it with the other species of Hydractinia. 



HYDilACTINIA DENDRITICA. 



(Plate II., figs. 7, 8, 9, 9a, 10.) 

 Zom/ia'e,?.— Specimen A: W.Q., March 21st, 1902; -10 fms. Specimen B: 

 Locality unrecorded. Label lost. 



VOL. III. 2 D 



