Prof. Allman on the Hydroida. $ 



wards the base it becomes smooth. An accumulation of thread- 

 cells takes place also at the tip of the tentacle, where they seem 

 to indicate a tendency to the formation of capitula, as in Con/Tie, 

 &c. The anterior tentacula, both by their structure and their 

 disposition in alternate series, remind us of the tentacula of 

 Coryne ; and I believe that we must regard them as the true 

 equivalents of the latter, while the posterior set must be viewed 

 as superadded structures. 



The endodermal lining of the polype-cavity, as far back as the 

 vacuolated portion, contains great abundance of brownish-red 

 pigment-granules, which are included in the inner cells of the 

 endoderm. The same pigment is continued into the stalks of 

 the gonophores, from which it may be traced into the manubrium 

 of the niedusoids, and, in their young state, into the radiating 

 canals. The whole of that portion of the body of the polype 

 which lies behind the posterior tentacula is destitute of pigment- 

 granules, and is accordingly colourless. 



The Stem. — The form of the stem is subcylindrical, usually, 

 however, enlarging towards the base, and again contracting and 

 tapering away to a blunt point. I have always found thia 

 pointed posterior part of the stem bent at nearly a right angle 

 to the rest, and, when the animal is in its natural position, 

 plunged into the sandy sea-bottom. 



To the naked eye, the stem is seen to be traversed from one 

 extremity to the other by narrow longitudinal bands or strije. 

 Under a low magnifying power, these bands are seen to inoscu- 

 late here and there with one another, while towards the base of 

 the stem they usually become broader and fewer by coalescence. 

 They owe their distinctness to the accumulation in them of 

 opake corpuscles. 



I have satisfied myself that the longitudinal bands represent 

 canals excavated in the endoderm of the stem. They lie just 

 within the ectoderm, while the whole of the axis of the stem is 

 occupied by a sort of pith composed of a large-celled, or, per* 

 haps, more correctly speaking, vacuolated tissue. Occasionally, 

 very distinct currents may be seen in the canals ; so that the 

 whole structure and phenomena presented by these parts closely 

 correspond to what we meet with in the stem of different species 

 of Tubularia. 



Under a high power of the microscope, delicate parallel longi- 

 tudinal strise may be detected lying external to the canals just 

 described. They are situated between the ectoderm and endo- 

 derm, apparently more intimately connected with the former, 

 and may be traced upwards on the body of the polype as far at 

 least as the zone of posterior tentacula. They seem to consist 

 of fine tubular fibres, and are apparently the equivalents of the 



1* 



