BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 353 



The absence of the proboscidian sheath and its contents leaves the central space 

 almost entirely at its disposal. 



In 0. alba and rosea Mr E. Ray Lankester* found many Gregarinse, but they 

 were rare in the specimens from St Andrews. In Tetrastemma varicolor a few 

 gregariniform parasites (Plate IV. fig. 12) occurred in the digestive cavity towards 

 the tail. 



Another parasitic structure was found in January in a large male specimen of 

 Ommatoplea alba in the form of an ovum enveloped in a granular lobulated 

 mass — lying close behind the ganglion of one side (Plate XIV. fig. 9, #), to the 

 exterior of the proboscidian sheath, and altogether unconnected with the oeso- 

 phagus. Externally there was a distinct hyaline capsule or cyst, to which 

 certain fragments of the fibro-granular lobulated covering adhered. The embryo 

 was furnished with a very conspicuous opaque granular mass, and two discs ; 

 while the general stroma was cellulo-granular, here and there closely streaked 

 by minute lines, apparently from its external investment. No motion of the 

 included animal was observable, except an alteration of the size and aspect of the 

 pores and discs after a period of eight or nine hours. There was no doubt as to 

 this being a Trematode-larva in its capsule, and by rupturing the latter a 

 complete view of the embryo was obtained (Plate XIV. fig. 10). The oral sucker 

 (c) was considerably smaller than the ventral (£), and this formed a marked 

 feature in the general aspect of the animal. The oesophageal bulb (d) appears as 

 a distinct swelling close behind the margin of the oral disc, and from the tube 

 behind the former the alimentary caeca (e, e) branched off and became lost in the 

 cellular tissues posteriorly. The opaque mass of cells and granules (at a) corre- 

 sponded to those observed in the Trematode-larva of the Carcinus mmnas,-\ 

 though, from the immature condition of this example, these and other structures 

 were much less definite. There were also two large circular granular bodies 

 (generative organs) (/ and g) ; but only a trace of the excretory tubes existed in 

 front near the oral sucker. 



Microscopically, the alimentary organ has scarcely the regular and firm 

 glandular appearance of the same structure in Borlasia, but is more friable and 

 cellular. Its analogy with that of the higher annelids is also borne out; for, 

 although the biliary matter is not arranged as a distinct organ exterior to the 

 alimentary, it is incorporated therewith, and probably has a similar function. 

 The fluid, however, which bathes the liver in the higher forms (if we suppose that 

 inside the sheath for the proboscis to be the homologue of the former), is here 

 separated by the muscular walls of its special tube. The large size of the pro- 

 boscis in the Ommatopleans renders this system very obscure from the dorsal 

 aspect, and it is only when the ventral surface is upturned that a correct know- 



* Jour. Micros. Sc. 1865. f Jour. Micros. Sc. vol. v. N. S., pi. viii. fig. 5, k. 



VOL. XXV. PART II. 4 X 



