DEPOSITION" OF OVA AND DEVELOPMENT. 117 



13. Mode of Deposition of the Ova. 



Instead of being deposited as free circular bodies, the ova in Lineus gesserensis are placed 

 within a flask-shaped membrane, with one end narrowed to a fine point, and the whole enclosed 

 in a tough covering of gelatinous mucus, which is fixed either to stone or glass, in the form of a 

 bulky cord, as noticed by (Ersted. When a female specimen is about to spawn, she seeks the 

 water-line, or a space above it, and quietly settles along the vessel. By-and-by a copious 

 exudation of tough translucent mucus takes place, which envelopes the entire animal. In this 

 mucus (Plate IV, fig. 3), which, when fresh, is crowded with small ovoid granular corpuscles from 

 the cutis, the ova are deposited in flask-shaped capsules, each of the latter corresponding to 

 an ovary, and containing all its ova, viz. from one to seven. Hence, by the nature of the parts, 

 the ova are arranged in a somewhat irregular double row along each side, the extremities of the 

 cord — corresponding on the one hand to the head and oesophageal portion of the digestive tract, 

 and on the other to the extreme tip of the tail — being free from ova. In some instances the 

 posterior end of the animal is curiously frilled and grooved on the ventral surface during 

 deposition. When newly exuded the mucus is softer and less tenacious than it afterwards 

 becomes, and the same may be said of the membranous flasks. The solidification of the mucus is 

 analogous to what takes place, under similar circumstances, in the egg-capsules of certain 

 mollusks, e.g. Buccinum undatum. If one end of the animal be disturbed from its original site 

 on the glass before the ova are all deposited, four rows will be found instead of two, for 

 sufficiently obvious reasons. The ova of Lineus gesserensis are of two shades, viz. white and pale 

 brownish ; and though the dark greenish examples often lay white eggs, they do not seem to do 

 so always. Each ovum measures from -^th to -^th of an inch in diameter. The deposition 

 takes place in January and Eebruary in those long confined ; but some specimens sent from the 

 rocks at St. Andrews towards the end of April likewise deposited ova, so that some latitude in 

 regard to date is necessary. The American examples spawned in January, and those from 

 Cuxhaven in March ; but the Nemertes communis of M. van Beneden only did so in September. 

 It is often observed that impure water causes recently captured animals to lay their ova rapidly, 

 as if from a kind of abortion. 



14. Development. 



The development of the ova of Borlasia obscura — a species apparently identical with our 

 Lineus gesserensis — has been described by E. Desor up to the period of the extrusion of the 

 young from the capsules; and Max Schultze and Krohn have also investigated the subject, 

 especially the former, so that I shall dwell only on such points as have not been elucidated. The 

 British forms seem to offer great facilities for these investigations, and I have had no difficulty in 

 rearing the Lineidce a long distance from the sea. 



The ova on deposition in the flask-shaped capsules (Plate XXIII, fig. 2) are uniformly 

 granular and opaque ; and when broken up, are found to be composed of a granular oily matter, 

 which forms streaks and rounded masses, and is not cellular, as described by E. Desor. The 

 clear, semi-transparent spot mentioned by the latter as occurring in the ova after deposition is 

 seldom visible, though the germinal vesicle [a) and dot (b) are apparent enough in the centre of 



