PARASITES. 129 



The large number of the Gregarince in some examples of the Nemerteans must give them 

 a position of importance in the economy of the worms. They likewise occur in the Planarians 

 and in the true Annelids. 



The small bodies shown in Plate XX, fig. 10, were extruded in multitudes with the 

 Gregarina from Lineus gesserensis and L. lacteus. They were generally of an ovoid or pyriform 

 shape — a few being circular, and contained many granules. Their diameter is about To^th of an 

 inch, or rather more. They appear to be pseudo-navicellee. 



Accompanying the gregariniform parasites certain ova are sometimes ejected from the 

 alimentary chamber, enveloped in mucus, and in the form of an elongated cordon (Plate XX, 

 fig. 11), the latter being rather more than the breadth of two ova, which are loosely scat- 

 tered in the slightly granular gelatinous matrix. These ova (Plate XX, fig. 12) measure 

 about 4^0 th of an inch in diameter, and each contains an embryo that, for some time after the 

 extrusion of the egg, makes very evident movements. They have two coats, and the embryo 

 is finely granular, with a large pale nucleus. I have not seen the embryo hatched in a perfect 

 state, but it is probable that these ova are connected either with the parasite of the muscles 

 hereafter to be described, or with an unknown trematode-larva. 



Another curious parasite is found burrowing in the body-wall of Lineus gesserensis, its 

 presence being readily recognised by the perforated and honey-combed appearance of the dorsum 

 of the affected animal, whose textures seem to be the seat of the workings of a microscopic 

 Tomicus typographies. When highly magnified the affected region appears to be covered with a 

 vast network of pale, minutely granular channels, which contain numerous opaque ovoicl granular 

 masses. On rupturing the body of the worm a large number of the peculiar structures (Plate 

 XVIII, fig. 17) slide out of the channels, and swim through the surrounding water, generally, 

 though not always, with the upper end (in the figure) first. 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 striae, as in Opalina, 

 though in a much more minute degree. Anteriorly is a conical portion {a), composed of three 

 rather indistinctly marked segments. Two evident annuli {b) succeed, the posterior part of the 

 last being narrowed, so as to cause a constriction of the body-wall. Behind are six nearly 

 equal divisions (<?), each often appearing double, that is, has a broad anterior and a narrow 

 posterior annulus. The posterior region (d) consists of three indistinct segments. The body is 

 minutely granular throughout, and an internal cavity is 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 is observed at either end. The opaque ovoid granular bodies (Plate XVIII, 

 fig. 18), scattered profusely throughout the infected portions of the Lineus, are evidently early 

 stages in the development of this species, and they too are ciliated. On subjecting them to gentle 

 pressure (fig. 19) transverse segmentation is apparent, the number of segments varying according 

 to the degree of advancement. The parasites are very delicate structures, and in the free state 

 soon break up into cells and granules, after discarding their cilia as above mentioned. Transverse 

 section of the affected worms shows that they occur both in the skin and in the 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 somewhat difficult point to determine whether 

 the skin, muscles of the body-wall, or digestive canal, constitute the common area of this 



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