334 H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 



and swinging its body from side to side, with frequent jerks, in its effort to pass over 

 some obstacle. 



The body is colorless, and enclosed in an uncontractile, smooth integument. It has an 

 asymmetrically ovate shape, rounded behind, and rapidly narrowed anteriorly into an oblique, 

 truncate conical front. Dorsally it is convex (figs. 67, 68), but ventrally, i. e., on the reptant 

 side, it is concave on the right and in the middle, and so strongly incurved on the left, that its 

 sharp edge (t) reaches nearly to the median line. Beneath this inrolled border the enclosed 

 space (fig. 68. t 1 ) projects into the left side like a longitudinal, covered way. In front it is 

 very deep, but from that point going backward it narrows gradually, and finally, with the 

 inrolled edge, fades out at the posterior third of the body. 



The two flagelh (fljl 2 ) are as widely diverse in character and function as any two similar 

 organs in the whole group of Protozoa. The anterior one (fl) is, strictly speaking, the 

 prehensile organ, as well as the main propulsory agent. It is quite delicate, and tapers 

 gradually, from its subterminal base within the longitudinal covered way, to an extremely 

 fine tip. In point of length it varies from one half to two thirds longer than the body. It 

 is always carried in an extended position in front, and vibrates very actively, especially 

 during reptation. 



The posterior flagellar organ, or gubernaclum (fl 2 ), is from three to four times the length of 

 the body, and arises far from the front, in the deepest part of the covered way (fig. 68, t 1 ), 

 and immediately beneath the contractile vesicle (cv). It is therefore attached quite near to 

 the left margin of the body, and between the anterior and middle thirds. Its base, which is 

 applied very obliquely to its point of attachment, is quite broad, but it narrows rapidly 

 into a uniformly, but scarcely tapering lash, which always projects forward more or less, and 

 then curves backward and extends to a long distance behind. During reptation over 

 smooth surfaces it lies along the abdominal, median line, and trails behind in long gentle 

 undulations. Although it never vibrates, it frequently lashes about, and applies itself against 

 obstacles on the right and left, or even in front, and acts as a prop upon which the body is 

 thrown from one side or the other, according to varying circumstances. That it is contrac- 

 tile would seem incontestible upon observing the sudden jerk with which it sometimes draws 

 the body back toward its distal end ; but I am pretty well convinced, from a careful study 

 of this movement, that, although this organ may be to a slight degree resilient, it is not truly 

 contractile, but rather flexible, and exhibits its muscular power by bending itself into coils or 

 zigzags. Occasionally specimens are met with which have an additional pair of flagella (fig. 

 69, fl 3 ), of a more delicate kind, attached near the others. That these originate as a pre- 

 liminary step to fissigemmation, although that phenomenon was not witnessed in this case, 

 there can be scarcely a doubt, inasmuch as it accords perfectly with what has been observed 

 in Anthophjsa (p. 329). 



The mouth has not been demonstrated to a certainty, by actually seeing food pass into it, 

 but an approximative determination was reached by observing particles of matter, which 

 were brought down by the prehensile flageUum (fl), pass into the body somewhere near the 

 front, and apparently within the compass of the covered way. 



The anus (figs. 65, 66, a) was adjudged to be at the posterior end of the animal, by notic- 

 ing, in a couple of instances only, a clear, more or less irregular, rounded mass in this region, 

 and its final disappearance while under observation ; but the substance was so transparent 



