260 HYDROID^. Part IV. 



These facts are higlily important with reference to the appreciation of the various 

 kinds of individuals found in the colonies of certain Siphonophoraj, in which a 

 medusoid form is often foimd combined with the structure of a hydroid. 



The spermaiic particleH. — During the earlier stages the mass of spermatic par- 

 ticles is perfectly hyaline, and, to all appearances, homogeneous ; but when the 

 medusoid is two thirds grown {^Fig. 15), the mass (c) is yellow, and composed, in 

 a great measure, of small cells {Fig. 15% h\ each one of which is filled by three 

 or four mutually comjii-essed, rounded angular bodies. As the mass comes to 

 maturity it assumes a dense yellow color, and the cells disappear, while their 

 contents are scattered uniformly throughout the field. By this time these angular 

 bodies have changed and become pear-shaped, and keej) up a constant agitation 

 among themselves. If the medusoid is opened and the mass torn, these lively 

 bodies {Fig. 16, A, B) run out and swim about the field of the microscope by 

 the help of a very long, slender, thread-like appendage (h), which is attached to 

 the narrower end of the pear-shaped head («) of the spermatic particle. 



Histologg. The adult Hydroid. — The cells, of which the tentacles of the lower 

 row are composed, are remarkably conspicuous, and the walls are so distinct that 

 they may be easily seen under a magnifying power of no more than one hundred 

 diameters. Seen thus, they appear like coarse, polygonal granulations, closely 

 packed together. Under a magnifying power of five hundred diameters their true 

 nature is revealed. Their superficial ends (PI. XXIIP. Fig. 1, c <?■ (? <?) are irregu- 

 larly polygonal, with an average diameter of about 2 sVo ^f ^"^ inch. They appear 

 much darker at this point, at the surface of the tentacle, because, beside the 

 numerous coarse, dark, angular granules, which are distributed throughout the body 

 of the cell, each one has one, two, or three lasso-cells fixed just below the surface 

 of the outer end. The lower side {Fig. 1, c^) of the tentacle is much more 

 thickly beset with lasso-cells than anywhere else. From this point, toward the 

 upper surface, they diminish in numbers; at the sides (c^) they are alreadj^, com- 

 paratively, vei-y few, and above (p) they are least numerous. In a profile view 

 these cells at once exhibit a marked difference, according to their position. At 

 the lower side {Fig. 1, «^ a^) they are at least one quarter longer than those on the 

 upper side (a), but not broader, whilst those at the sides {Figs. 1 and 3, (?) are much 

 shorter than anywhere else. Each cell occupies the whole thickness of the wall 

 {Figs. 1, 2, and 3, from li} to ci^\ and is wedge-shaped, with the broader end («') 

 outermost. The inner end («^) is quite sharply ti'uncate, and presses closely upon 

 the even surface of the inner layer {e). On this account the line of demarcation 

 between the two walls is very sharply defined. The outer ends of the cells are 

 variable in shape ; when the tentacle is stretched to its fullest extent they are 

 not in the least protuberant {Fig. 1, h I?-), but when the tentacle is less extended, 



