124 MORPHOLOGY. 



The endoderm of the proper digestive cavity is probably in all Hydroida thrown into 

 more or less complicated lobes or ridges, which disappear where this cavity passes into the common 

 cavity of the coenosarc. 



This condition may be well seen in Ili/drafusca, especially when, as sometimes happens, we 

 have an opportunity of looking down into the stomach of the uninjured animal through the widely 

 open mouth. Numerous ridges may then be witnessed, extending from the walls of the cavity 

 almost to its centre. These ridges are rendered papillose by the projection from their surface of 

 the peculiar piriform cells already described, while the furrows between them are comparatively 

 smooth. I have witnessed a similar condition of the endoderm in the digestive cavity of 

 Cordyhphora lacustris, Coryne pusilla, St/ncoryne eximia, and many others, where thick, irregularly 

 sinuous and lobulated ridges of endoderm project into the stomach from the inner surface of its 

 walls. A furrowed and lobulated state of the endoderm may also be witnessed in the manubrium 

 of the medusa, and in the spadix of the sporosac of many hydroids.' 



The hydranth of Tubularia indivisa presents a very remarkable condition of its endodermal 

 layer (PI. XXIII, fig. 1). Immediately within the mouth, the cells containing the coloured granules 

 form a narrow smooth zone of vermilion dots, immediately behind which the surface of the endoderm 

 is disposed in irregidarly oval prominent vermilion patches, separated by paler narrow sulci. In 

 tracing these patches backwards, we find that they become smaller and more numerous, gradually 

 resolving themselves into small spots, and ultimately into minute scattered puncta which at the 

 posterior extremity of the hydranth, where its cavity passes into that of the stem, become 

 more densely grouped, and are here arranged in radiating bands of a bright vermilion colour. 

 Along the line where the anterior contracted portion of the hydranth passes into the wider 

 basal portion, the endoderm is thrown into numerous remarkable pendulous lobes of a piriform 

 shape and bright vermilion colour (PL XXIII, fig. 3). They consLst "each of a cluster of very 

 distinct spherical cells, containing vermilion-coloured granules, among which are numerous small 

 clear spherical elements, apparently oil-drops. They present a close resemblance, both in their 

 form and in the nature of their contents, to the zone of gland-like lobes which occupies a very 

 similar position in the interior of the hydranthal zooids of certain Siphonophora. 



Canaliculation of endoderm. — In some Hydroida we have a very peculiar and exceptional 

 condition of the somatic cavity and of its endodermal lining ; for while this cavity in most 

 Hydroida consists of a simple tube, it is in the cases here alluded to composed of numerous 

 intercommunicating canals. 



This condition is well seen in different species of Tubularia. The stem of Tubularia indivisa 

 for example, presents immediately within the perisarcal tube a continuous layer of ectoderm 

 enclosing the endoderm which extends to the very centre of the stem, and thus obliterates all trace 

 of a central somatic cavity (PI. XXIII, fig. 7). The place of this cavity, however, is suppHed 

 by numerous canals, which are excavated in the endoderm and take a longitudinal course through 

 the stem, occasionally communicating by lateral offsets with one another, and finally all merging 

 in a common central cavity at the base of the hydranth. 



In a transverse section of the stem, the canals present a wedge-shaped form, the narrow end 



* In certain Geryonidce (Glnssocodon eurylnu, Carmarina hasiata), Haeckel (op. cit.) describes, as 

 gastric glands, peculiar leaf-shaped organs which occur in the inner walls of the manubrium (four in Glosso- 

 codon. six in Carmarina). They are composed of clusters of large cells with dark-coloured contents. 



