Chap. III. GENUS PLEUROBRACIIIA. 215 



to appear to be specially fitted for the movement of any particular organ. Yet 

 the cellular system is not a confused mass, as perhaps might be inferred from tlie 

 foregoing remarks, but these gigantic vesicles, reminding one of the fusiform pulp- 

 sacs of the orange both by their shape and disposition, have a most beautiful 

 and extraordinary arrangement, which is thus far only known among Ctenophora\ 

 In order that there may be no misapprehension in regard to the nature of 

 this extensive naotory system, it is advisable to render its cellular character as 

 distinct as possible in the mind of the reader, and therefore, Ijefore proceeding 

 further, we will describe the cells in detail. We have already remarked on their 

 extreme transparency : this, combined with their great length and the prominence 

 of their walls, in profile, conspires to obscure their true character, and suggests the 

 idea, even to the oljserver well acquainted with them, that they are a network 

 of widely separated fibres ; and as such they have been described in a previous 

 memoir,^ though their vesicular character was already observed to some extent. 

 They are in reality elongate, irregularly spindle-shaped vesicles {Fig. 24), either 

 with blunt ends («) or with all degrees of acuteuess to quite slender, pointed 

 terminations {e). When seen from either the actinal or the abactinal pole of the 

 body, they generally display a broader outline than from any other point; and, as 

 the eye passes along the sides towards the middle region of the body, they present 

 successively nari'ower and narrower boundaries, until, at a right angle with the first 

 point of vieAv, they show a minimum of lireadth. Viewed transversely, they are 

 irregularly j^olygonal, and on that account each cell appears to have three, four, 

 or five narrow ribs, trending parallelwise to its longer axis, and corresponding to 

 its angles. When in a state of contraction, the walls are wrinkled transversely 

 to the longer axis {Fig. 24 5 <?) ; but, owing to their transparency, the angles alone 

 are readily seen, and they appear like thin, tortuous fibres, or oftentimes, strange 

 to say, as if they were spiral springs, adapted to keep the body in a state of 

 tension. This false appearance is not easily disjielled if it impresses the mind at 

 the first view of these cells, especially as it is very difficult to trace the true 

 relations of their sides and angles, without having some sort of hint as to their 

 true nature. The total absence of the usual cell constituents — mesoblast and more 

 or less granular contents — contributes also to keej) up the illusion ; for every thing 

 within is as clear and homogeneous to the eye as blank space itself Indeed, 

 any one, after remarking the glass-like transparency of these animals, would be apt 

 to doubt whether any structure could be detected in them. Such is the almost 

 etliei-eal nature of the motive power Avhich governs the actions of Pleurobrachia. 



^ Contributions to the Natural History of the American Academy of Arts and Sciences, Boston, 

 Aealephaj of North America, by L. Agassiz, Mem. Mass., vol. 4, jiart 2, 1850, p. 330. 



