74 ANATOMY OF THE ENOPLA. 



d. Reproduction of the Proboscis. 



This has been observed in various species. In a specimen of Tetrastemma melanocephala, 

 from which, three days before, the proboscis had been removed, there existed a pale conical 

 papilla, which projected a short distance behind the ganglionic commissures. Two days after 

 considerable progress has been made, and the organ proceeds backwards as a slender rod — tapered 

 posteriorly (Plate XIII, fig. 2, a). There is a distinct exterior coat from one end to the other, 

 and an inner — terminating at the commencement of the posterior narrow portion. The former has 

 a crenated edge in contraction. The organ gradually increases in size and complexity, but con- 

 tinues quite free posteriorly for a considerable time, until, indeed, the stylets are well developed. 

 At a further stage (Plate XIII, fig. 3), the walls are defined almost as in the complete structure, 

 but of course are much more delicate and plastic ; and the extreme contractility and elasticity of 

 the entire organ are most interesting, and raise a doubt as to the identity of the muscular fibres 

 with those of the higher invertebrates, since they so much surpass them in mobility. The floor of 

 the anterior chamber ends in the usual pit, which is dilated on account of the shortening of the 

 organ. The walls of the muscular cavity behind the floor of the anterior region are not well 

 defined, though the space itself is large, and contains a granular fluid. There is no central 

 stylet, and the basal apparatus is represented by a somewhat triangular group of the usual granules, 

 round which the radiating fibres are placed. The wedge-shaped investment within the latter 

 (fibres) is mobile and translucent. A somewhat indistinct streak (/) in the median line indicates 

 the canal for the central stylet, and now and then this is bulged by projected fluid. The marginal 

 stylet-sacs, from the expansion of the chamber in this instance, seem to be carried backwards, but 

 in reality they have their distinctive position. Each contains a stylet or two, a few granules, and 

 a clear globule. 



The reservoir at this stage has assumed its characteristic shape, though the glands are 

 barely visible. The contraction of the anterior and posterior chambers has annihilated 

 the usual prominent appearance of this part, and the last has encroached very much on the cavity 

 posteriorly. The glands are formed in the posterior chamber, though their contents are not 

 elaborated, and the cavity terminates in the usual cul-de-sac, A few rounded papillae at the 

 termination indicate the early condition of the muscular riband. It is clear that at some time 

 or other the latter becomes attached to the wall of the proboscidian sheath, and that, too, in a 

 definite manner, since no great deviation in a series of specimens is met with. 



In the developing organ of N. gracilis (Plate XIII, fig. 17) a very good analysis of the 

 somewhat complicated structure is obtained, so that doubtful anatomical points are cleared up 

 satisfactorily. The apparatus at the base of the central stylet is sometimes composed of 

 granules in rounded masses ; and they are all grouped posteriorly at an early stage, thus 

 presenting a similar form to that seen in other species which have short structures in the 

 complete state. It is curious to witness the accuracy with which the stylets are reproduced in 

 this and other species. There is never any confusion, but each invariably develops them of their 

 respective sizes and curves as infallibly as if they had been struck out of the same mould. Yet 

 these bodies are not in any way organically connected with the tissues of the proboscis, but only 

 spring from a secretion poured into the marginal sacs, or from the central apparatus. In the 

 concentric arrangement of their constituent substance, and some other particulars, these spicula 



