2 Prof. AUman on the Hydroida. 



count of the structure of this Hydroid, to divide the animal into 

 the polype, the stem, and the gonophores. 



The Polype. — The polype may be described as flask-shaped ; 

 it bears towards its base a zone of about thirty-two long and 

 imperfectly contractile tentacula arranged in a single series, 

 while at a considerable interval in front of this zone, and a little 

 behind the oral extremity, is a brush-like group of about eighty 

 very contractile tentacula, much smaller and finer than the 

 posterior, and arranged in six or seven closely placed alternate 

 series. 



Immediately within the zone of posterior tentacula are the 

 branched stalks of the gonophores ; their axis is occupied by a 

 continuous tube, which communicates freely with the cavity of 

 the polype, and they carry the gonophores in clusters upon the 

 extremities of the branches. They are usually from fifteen to 

 twenty in number, arranged in two alternate series. The gono- 

 phores will be afterwards described. 



When a longitudinal section is made through the polype from 

 the mouth to the stem, it will be seen that the body, as far back 

 as the zone of posterior tentacula, presents a continuous cavity, 

 with the endodermal lining of the narrow anterior portion thrown 

 into prominent rugae, and with the floor of the cavity projecting 

 as a broad conical elevation into the wide posterior portion. 

 This conical projection consists of a very much vacuolated endo- 

 derm ; and the same vacuolated structure is continued backwards 

 as far as the origin of the stem, giving to the whole of the pos- 

 terior part of the polype the appearance of being filled with the 

 vacuolated tissue, and destitute of any distinct cavity. A care- 

 ful examination, however, will show that this vacuolated mass is 

 perforated in its axis by a tubular prolongation of the cavity of 

 the polype, though, in consequence of temporary obliteration by 

 the approximation of its walls, this continuation of the polype- 

 cavity is usually very difficult to detect. It is continued to the 

 summit of the stem, and then, becoming somewhat wider, re- 

 ceives the longitudinal canals of the stem, to be presently de- 

 scribed. 



The posterior tentacula are destitute of any trace of a cavity, 

 and consist of a simple prolongation of the vacuolated endoderm 

 of the body, surrounded by a layer of ectoderm, the endoderm 

 becoming somewhat closer in texture as it enters the tentacula. 



The anterior tentacula seem to admit the cavity of the polype 

 for a short distance into their interior ; but their tube soon be- 

 comes obliterated by the vacuolation of their endoderm, which 

 assumes the usual septate appearance. For some distance from 

 their extremities the ectoderm is thrown into slightly elevated 

 verruca in which minute thread-cells are accumulated, but to- 



