126 A NEW FAMILY OF HYDROIDEA. 



Thickness of wall of tube, -0525 mm. at the exterior, some of those in the 

 centre being not more than -00525 mm. 



Hyclrotheca. — Transverse section, -175 mm. — -1225 mm. 



Hydrotheca. — Longitudinal section, "525 mm. (avemge). 



Soft Parts. (Figs. 3, 4, 6, 10, 12, 14, 16). 



The chitinous tubes are filled by the ccenosarcal tubes which branch and 

 anastomose freely. No one of the latter tubes is more prominent than any 

 other. The polypes are distributed over the whole surface, having apparently no 

 definite arrangement on the larger stems or branches, but, in the smaller ones, 

 a very distinct spiral arrangement (Figs. 6, 11). One corresponds to each of the 

 hydrothecae, seen in the skeleton, and is capable of complete retraction within the 

 latter. When thus retracted the opening is protected by the thin collar -like projection 

 formed of the outer layer of perisarc mentioned above. 



Each polype has the typical hydroid form. It is somewhat tubular, with a 

 conical hypostome, from the base of which arises a single circlet of solid tentacles, 

 which vary in number from six to ten, and are provided with minute nematocysts, 

 the threads of which are short, stiff, and unbarbed. When the tentacles are retracted 

 they curl over towards the mouth, much in the same manner as do those of 

 Pedicellina, amongst the Polyzoa. 



The body of the polype is formed of the typical layers — an ectoderm of somewhat 

 columnar cells, an endoderm of larger cells less defined in shape, and between the 

 two a very strongly marked layer of mesogloea (.00876 mm. in thickest part). From 

 the bases of the ectoderm cells pass off long processes (Fig. 14), much more 

 strongly marked than is usual in hydroid forms. In its greatest breadtli one of 

 these fibres measures -0058 mm. They give rise to a circular band of what are 

 evidently in function, " muscle fibres." In figure 14 these are represented as seen 

 when an oblique longitudinal section of a polype is cut.* Possibly some of them 

 may have lost their connection with the ectoderm cells from which they have been 

 derived. Each has a very definite outline and contents, which, -when stained, have a 

 somewhat granular appearance. They all lie external to the mesogloea, and form a 

 very definite band round the body of the polype, the position of which is represented 

 in the diagram (Fig. 12). 



Sections, transverse and longitudinal, show that the basal ends of the polypes 

 are continuous with more than one (sometimes as many as four or five) of the 



* These benr a considerable resemblance to those described by Weiamann as occurring in the coenosarcof Plumularia 

 echinulata. See Entstehung der Sexualzellen bei den Hydromedasen, PI. VIII., Fig. 96. 



