LUCEIINAK1.-E AND T 11 E 1 U ALLIES. 93 



CHAPTER VI T. 



HISTOLOGY OF II A L I C L Y S T U S AURICULA. 



§ 25. The Umhdlar and Pednncu/„r WalJs. 



191. In some respects it would he better to connect tlie histoloijy of tlie parts 

 and organs of the body witli their general structure, so tliat all their characteristics 

 might be presented in one view ; but tliere are other and weightier reasons why a 

 separate chapter should be devoted to the intimate structure of the tissues. In tlie 

 first place the subject and arrangement of the sections are rendered nuu-h less com- 

 plicated and clearer for comparison; and again, it leaves more room for the discus- 

 sion of collateral questions, such as the cell-theory, and the like, without involving 

 the confusion of parenthetical sentences and explanatory notes. Yet we shall so 

 arrange the sections that each may be read as a continuation of any previous one 

 devoted to the general description of a corresponding organ or part. In accord- 

 ance with this purpose we begin with the outermost wall of the anterior face of 

 the umbella. 



192. To those who accept the theory that the tissues of Acalephje, and other 

 groups of this grand division, are composed of true cells, wlierever tliere is the 

 appearcuice of such bodies, it will come with most force, when we state that in 

 Liicernarire, as in other orders of the class, there is but a slnrjle strahim of cell n in 

 each wall. If we except the family Tithidarulrp, tliis may be announced as a lam 

 among all Acale2)ha\ both in the liydroid as well as in tlie medusoid morpli. In 

 most highly developed medusoid morplis these strata are so excessively tliin, tliat 

 is to say, the component cells are so sliallow, tliat they have more of the character 

 of a filmy epithelium than of a true wall ; whereas in tlie hydroid morphs it fre- 

 quently happens that some one of the layers is so thick as to constitute three- 

 quarters or more of the whole bulk of the body. In ]>ucernaria3 we find these two 

 extremes indiscriminately intermixed. 



193. The oprnphrarfma (see ^61, 62) of the umbella ( /'V/s. 74, 85, 86, n-u^) is 

 composed mainly of the single stratum of cells which are so conspicuous, even to 

 a casual observer ; but these cells are l)y no means all tliat give bulk to this layer, 

 for they stand at more or less considerable distances apart from each other, and 

 the large interspaces that intervene add greatly to the physiognomy. In fact the 

 color of this region is almost altogether due to the large masses of pigment gra- 

 nules (e") which crowd these interspaces, and underlie the cells ; and th(> latter are 

 rendered all the more conspicuous by the dark ground-work in which they are 

 imbedded. The cells are not arranged collaterally in any particular order, nor 

 have they a unifornr size, some being at least twice, and occasionally three times 

 the diameter of others ; on the average they are about oni'-quarter deeper than 



