EEPOET ON THE HYDEOIDA. 
XXV 
though in a few its formation takes place on so insignificant a scale as to render its 
presence liable to be overlooked. In the Hydrocorallia it is replaced by an internal hard 
calcareous eorallum. In by far the greater number of cases it constitutes a firm pro- 
tective tube by which the ccenosarc is invested, while in the great primary section of the 
Calyptoblastea, it is continued for a greater or less extent, and in a more or less modified 
form, over the various zooids of the colony. In these it forms the cup-like receptacles or 
hydrothecse into which the hydranths can become retracted, as well as the peculiar 
receptacles — gonangia — destined for the protection of the generative buds (PI. X. fig. 2a; 
PI. XII. fig. la; PI. XXIV. fig. la; &c.). In the Gymnoblastea, on the other hand 
(PI. IIP), neither of those protective extensions of the perisarc exists. In two very 
remarkable genera, Synthecium and Thecoclaclium, fine examples of which have been 
brought home by the Challenger, another function besides that of the protection of the 
hydranth would seem to devolve on certain hydrotheeae. In Synthecium some of the 
hydrothecae situated on definite parts of the colony contain no hydranths, but on the 
other hand enclose, each, the peduncle of a gonangium which springs from the bottom of 
the hydxotheca (PI. XXXYII. fig. la, &c.). In Thecoclaclium, again, every branch of 
the colony springs from within a hydrotheca, which thus, instead of containing a 
hydranth, encloses as in a sheath the proximal end of the branch (PI. XXXVIII. fig. 3). 
The perisarc varies greatly in thickness, from a strong coat in which numerous layers 
of deposition may be seen, to a delicate, scarcely recognisable pellicle, and is invariably 
absent from those zooids which have detached themselves from the colony in order to lead 
an independent life in the open sea. 
While the coenosarc of the Hydroid colony is, as an almost universal condition, 
protected by an external perisarcal tube, an exception to this condition is found in 
Hydractinia and Podocoryne as well as in the entire section of the Hydrocorallia. In 
Hydractinia and Podocoryne the chitinous perisarc forms a continuous thick stratum 
permeated by a network of anastomosing coenosarcal tubes, and overlaid by a naked 
extension of the coenosarc. In the Hydrocorallia the calcareous eorallum, which here 
forms the hard skeletal tissue of the colony, and is also permeated by anastomosing 
tubes of coenosarc, is in a similar way overlaid by a superficial covering of coenosarc. 
Indeed the relations between the Hydrocorallia and Hydractinia are in many respects 
of the most intimate kind. 
V. The Gonosome. 
The various zooids and associated structures now described are more or less directly 
connected with the nutrition of the colony, and may, as has already been said, be 
collectively designated by the name of trophosome. There are, however, other parts of 
the colony on which a different group of functions — namely, that of sexual reproduction 
(zool. chall. exp. — paet Lxx. — 1888.) Aaaa d 
