362 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



considerable distance backwards by the side of the lateral nerve-trunk. Besides 

 these sacs there is in the snout of this worm a series of well-marked glandular 

 organs in front of the ganglia, one of which lies on each side of the blood-vessel, 

 and is connected with a large lobulated mass in the middle line. In structure 

 these glands are allied to the foregoing, having in their interior rounded 

 granular cells, pigment, and other granules. What in some views appeared 

 to be a duct passed from the posterior end of the external lobule towards the 

 cephalic sacs. Traces of similar glandular masses were seen in other species (e.g. 

 O. alba) near the middle line of the snout, behind the cephalic sacs, and else- 

 where. In Tetrastemma the sacs agree essentially in structure with those of 

 O. alba, and in such translucent specimens as T. varicolor the ciliated posterior 

 ducts are easily traced. 



The slight furrows just described on the head in this group have been noticed 

 by few investigators, and only Prof. Keferstetn* and M. Claparedej mention the 

 occurrence of the sacs; the former using the term Seitenorgane for their signifi- 

 cation, but his notice is very brief. He figures and describes his B. splendida as 

 furnished with sacs at the side of the ganglia, but without the ciliated ducts 

 posteriorly; while in B. mandilla the latter reach no further back than the 

 ganglia. The former species has a curious series of oblique furrows on the side 

 just behind the snout, which are evidently homologous with those described in 

 O. pulchra. M. Claparede again figures on each side of the eyes in the young 

 of ProsorJiochmus Claparedii a blind sac, apparently unconnected with the ciliated 

 pits above-mentioned : moreover, in the drawing of the adult animal (fig. 10) 

 there is on each side a ciliated duct, but no sac. M. de Quatrefages only noticed 

 traces of these structures in the Ommatopleans ; for he describes bridles or bands 

 as passing outwards to the " fossettes cephaliques." In his Polia bemUx he 

 represents a large nerve passing from the anterior part of each lateral nerve- 

 column, not far behind the ganglion, and which, after a course directed obliquely 

 forwards, ends in a swollen granular manner at the cephalic fossa. A similar 

 arrangement occurred in P. humilis; but in this instance the nerve arose from 

 the superior lobe of the ganglion, passed obliquely forwards and outwards, and 

 ended in several branches at the fossa. In Cerebratulus crassus and Nemertea 

 peronea, again, he figures the nerve as springing from the posterior part of the 

 superior lobe. He does not seem surprised that the nerve-trunks to these fossae 

 should spring from sites so diverse as the front and back of the superior lobe and 

 the lateral trunk. The disposition of an important nerve-branch in species of 

 the same genus, or even in allied genera, is seldom so varied. The structure 

 appears to have been misinterpreted in Ommatoplea, the sac having been over- 

 looked, and the process or duct, which sometimes crosses to the origin of the 

 great nerve-trunk and ganglion of its side, assumed to be a nerve-branch. M. 



* Zeitsch. fur wiss. Zool. xii. 1863, pp. 81 and 82. f Beohach. fiber, &c. pi. v. fig. 12. 



