no MORPHOLOGY. 



it as a naked prolongation, reminding us of the naked prolongation of the spadix in certain 

 Aracece. 



In Clava sqtianiafa (PI. I, figs. 1 and 3) and in CJavamulticonm (PI. II, fig. 1, and woodcut, 

 fig. 47) the gonophores form dense clusters surrounding the hydranth in a sort of verticil. Each 

 cluster consists of sessile gonophores borne on a greatly depressed common peduncle, and thus 

 recalling the form of inflorescence known as a capitulum. The order of development, however, 

 appears to be centrifugal, instead of being, as in the true capitulum, centripetal, and would 

 therefore, perhaps, more truly suggest a comparison with the depressed cyme which constitutes 

 the axillary inflorescence in many Lahiato'.. 



In the comparison just instituted between the gonosome of the IItdroida and tlie inflores- 

 cence of plants it will be noticed that, whenever in the Hydroida the generative buds are borne 

 upon a special gonosomal axis like the flov/ers in an inflorescence, the order of succession is far 

 more frequently a centrifugal than a centripetal one. In the calyptoblastic forms, indeed, it is 

 always centrifugal. This is exactly the opposite of what prevails in plants, for here the centripetal 

 forms of inflorescence greatly exceed the centrifugal ones. 



We must be careful, however, not to assign to the resemblances which may be noticed more 

 importance than they are justly entitled to. Yet, after setting aside such as are merely superficial 

 and accidental, many still remain which have their origin in certain deep-seated properties. They 

 may be referred to the common phenomenon of gemmation, which, by agamic multiplication in the 

 animal as well as in the plant, gives rise to colonies whose members, in each case mutually 

 dependent on one another, continue to be organically associated into definitely arranged and 

 determinate groups. 



V. Histology. 



Both trophosome, gonosome, and coenosome have now been considered in their broader 

 morphological features ; their morphology, however, cannot be regarded as complete without some 

 further anatomical details, embracing a histological examination of the tissues. 



The Hydroida possess an exceedingly simple structure. Every hydroid, as we have already 

 seen, is composed of two layers, an ectoderm and an endoderm. Each of these may present in 

 itself various degrees of differentiation, and we can, perhaps, best study the minute texture of the 

 tissues by examining them first in the ectoderm, and secondly in the endoderm. 



1. Tlic Ectoderm. 



General sfructiire of Ectoderm. — In many cases it is impossible to detect in the ectoderm of 

 the mature hydroid any well-defined structure. A homogeneous blastema with granules and 

 scattered nucleus-like corpuscles, and thread-cells more or less thickly immersed in it, are all 

 that the microscope has as yet in such cases succeeded in demonstrating, though a celliUar struc- 

 ture of this layer can almost always be observed in the embryo. 



