BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 387 



November. The animals infested by this parasite present a remarkable aspect, 

 the posterior half of the dorsum appearing under the lens to be honey-combed 

 and tracked by pale channels in every direction, as if a microscopic Tomicus 

 typographies had been at work in their bodies. Under the microscope the vast 

 net-work of pale channels have a minutely granular appearance, and numerous 

 small, opaque, ovoid granular bodies likewise occurred. Upon rupturing the body 

 of the worm, a large number of the peculiar structures (Plate XII. fig. 4) slid out 

 of their investments, and sailed about in the surrounding water, generally, 

 though not always, with the upper end in the figure first. They differed totally 

 from the gregarinse above-mentioned, many of which, however, were present in 

 the same hosts. Externally, they are coated with long cilia, whose activity in 

 the free state is of somewhat short duration, for after a time the animals remain 

 quiet, and they drop off. The body is distinctly segmented, and tapers slightly 

 towards the posterior end ; while the surface is marked by very fine longitudinal 

 lines, as in Opalina, though in a much more minute degree. Anteriorly, there is 

 a conical portion (a), composed of three rather indistinctly- marked segments. 

 Two well-marked annuli (6) succeed, the posterior part of the last being nar- 

 rowed, so as to cause an evident constriction of the body- wall in many positions. 

 Behind these are six nearly equal divisions (c), each of which often appears 

 double, that is, has a broad anterior and a narrow posterior belt, as indicated in 

 the figure. The posterior region (d) consisted of three indistinct segments. The 

 body was minutely granular throughout, and an internal cavity was apparent 

 from the fourth segment to the last ; commencing in the former by a rounded end, 

 and terminating just within the border of the latter. No aperture was observed 

 at either end. The opaque ovoid granular bodies (Plate XII. fig. 6), scattered pro- 

 fusely throughout the infected portions of the Borlasian, were evidently young 

 stages in the development of this species, and they too were ciliated. Upon subject- 

 ing them to gentle pressure (Plate XII. fig. 5), transverse segmentation was appa- 

 rent, the number of segments varying according to the degree of advancement. The 

 parasites were very delicate structures ; and in the free state soon broke up into 

 cells and granules, after discarding their cilia as above-mentioned. Transverse 

 section of the affected animals showed that they occurred both in the skin and in 

 walls of the digestive tract ; their ravages in the pigmentary layer of the former 

 tissue causing the curious appearances which led to their detection. It is a some- 

 what difficult point to determine whether the skin, muscles of the body- wall, and 

 the digestive canal, constitute the common area of this creature's depredations ; 

 or if it was piercing the former on its way to the surface, or again passing 

 towards the alimentary cavity to be voided per anum. The differently seg- 

 mented condition of the full-grown specimens, and their internal structure, 

 exhibit a higher type of organisation than the ordinary Opalina and Pachyder- 

 mon, which again are more elevated than the Gregarinse. The ease with which 



