This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(698). The segmental organs in the genus Terebella, in all essential particulars, agree in their general structure with those of Arenicola. They differ in number in different species: thus, in Terebella nebu-losa there are sixteen pairs, in T. conchilega only six, and in T. multi-setosa twenty-four. The testes in the male Terebella, according to Dr. Williams, are the lateral pouches or true segmental organs; in the female, these are the ovaria: the generative products in both sexes are early introduced into the general cavity, in the fluid of which they become rapidly developed. There exists, however, in the Terebellae a large glandular mass extending from the head, along the median line, to some distance in the direction of the tail. This glandular-looking organ coincides internally with the smooth, foot-like, dense tegument-ary structure observable on the thoracic half of the abdominal aspect of the body externally, and has been described by Cuvier, Milne-Edwards, De Quatrefages, Grube, Stannius, and others as the testes. It would seem however, from the researches of Dr. Williams, that they have nothing to do with the generative system. They are present alike in the male and in the female.
They consist of follicles filled with large fatty particles; and their office seems to be to supply the lubricating and cementing fluid by which the animal forms and moulds its tube.
(699). In these Erratic Annelidans, according to M. de Quatrefages*, the eggs, as well as the spermatozoids, which exist in a very rudimentary condition in the ovary or the testicle, break loose into the abdominal cavity, where, insulated from all the solid parts, and without any connexion with the vascular system, they undergo all the principal phases of their development. "It appears," says M. de Quatrefages, "that the liquid which thus bathes them on all sides must be vitalized, and that it is from it that they receive the materials necessary to enable them to grow to ten times their original size; in fact, this fluid acts the part of an ovary and of a testis to them. The liquid enclosed in the general cavity of the body of the Annelidans is therefore in some respects a fluid organ".
* "Sur le Sang des Annelides," Ann. des Sci. Nat. 1846.
(700). The spermatogenous masses floating in the fluid contained in the general cavity of this Annelid are irregularly ovoid, and present themselves, as is usual, in different stages of development. At first they are perfectly diaphanous, smooth, and manifestly homogeneous, without any trace of an enveloping membrane. The dimensions attained by them in this condition reach to as much as 1/16th of a millimetre in length, and 1/23rd of a millimetre in breadth.
(701). At this epoch they may be seen to exhibit two grooves, crossing each other at a right angle, and whose direction does not appear to present any constant relation with the form of the mass itself. The number of grooves soon increases, and they become more marked and deeper, and the mass, after having presented a surface subdivided into large irregular lobes, assumes a mulberry-like aspect, and ultimately becomes completely granular. During the time that these phenomena are being manifested, the mass continues to increase in volume, and in its ultimate condition it is sometimes 1/12th of a millimetre long by nearly 1/16th of a millimetre broad.
(702). The masses, when a little further advanced, split up, and the tail of the spermatozoids is then apparent. The spermatozoids continue to adhere together for some time longer by their bodies, as well as to the granulations not yet transformed: ultimately they are gradually separated.
(703). At the moment when the spermatozoids separate themselves from the minute masses of which they constitute a part, their body is almost fusiform, and perhaps not more than 1/10th of a millim. long and 1/300th of a millim. thick; but they grow during the time they remain in the fluid that bathes them: the body and the tail elongate, and, besides this, the former increases considerably in its transverse diameter. Among spermatozoids quite mature, some will have attained to a length of 1/60th of a millim., and a breadth of 1/150th of a millim.
(704). According to Dr. Williams*, here, as elsewhere throughout the class of Annelidans, the segmental organs are to be regarded as the primary parts of the reproductive system. Throughout the Nereid group, the ciliated, horseshoe-shaped segmental organ exists. It consists of a tube, highly ciliated, both ends of which communicate with the exterior: the ingoing limbs are situated in the immediate vicinity of each dorsal foot; the outgoing limbs, considerably longer and more tubular than the former, open externally to the median side of the root of each ventral foot. The cilia with which this horse-shoe tube is lined are highly vigorous, and capable of supporting a powerful current, which arises externally and terminates externally; but, as Dr. Williams asserts, the ova in the female and the sperm-cells in the male escape, although in some undetermined mode and by some undemonstrated passage, from this organ into the complexly areolated tissue which fills the chamber of the pedal appendages, which is a development of the segmental organ, and in size and vascularity is proportionate to the stage at which the contained germinal elements have arrived.
* Phil. Trans. 1858, p. 124.
The following observations of M. Sars*, relative to the embryogenesis of these worms, are extremely interesting and important: endowed with lively motion. They are extremely unlike their parent both in form and structure, being short, oval, cylindrical, and devoid of segmentation, furnished with a circle of long cilia around the centre of the body, but otherwise without external organs. The portion of the body situated anterior to the ciliary circle is somewhat narrower than the hinder one, and bears two eyes: this is the head; and the young one always swims with this extremity in front. Frequently these young animals revolve, during swimming, around their longitudinal axis. Their sight is distinctly developed; for they avoid each other with adroitness, and always swim towards the light. The time from the extrusion of the young to the laying of the eggs may probably amount to a couple of weeks.