From this portion of the tube they escape by the external orifice.

(637). The testes on the other side bear the same relation to the tube as the ovary. The utriculus upon one side is represented by the ejacu-latory pouch on the other. At present it is beyond the power of science to explain why these organs, in only one or two annuli of the body, should be implicated in the sexual development, while all the rest remain in abeyance in the condition of mere 'excretionary tubes'; or why the ciliated tube of one side should be changed into the female system, and that of the other into the male.

(638). Besides the ordinary mode of propagation by ova, it has long been asserted that some of the Annelida, at least, are reproduced by spontaneous division. Bonnet, Muller, and Duges all agree that this is the case with certain species of Nais; and in Nais filiformis the process of separation has been witnessed from its commencement to its termination. The division was said to occur near the middle of the body of the animal, the posterior half remaining motionless upon the mud of the bottom of the vessel, whilst the anterior portion buried itself as usual: after some days the truncated extremity of the hinder part was observed to become swollen, transparent, and vascular, and ultimately to assume the complete structure of the mouth of the perfect animal; it then buried itself in the mud, and no doubt there completed its development.

The following is what Dr. Williams conceives to be the interpretation of the above facts: -

(639). Every Nais, as relates to its reproductive apparatus, is identically constituted, and this worm is consequently androgynous. Every individual, towards the latter end of the summer, dies by the bisection of its body. It is not true, as reported by Duges, and before him by Spallanzani, that the fragments into which the body of each worm becomes resolved are again reconstructed into a perfect whole. Although the sexual system exhibits a tendency to segmental repetition, there devolves upon the large anterior portion, described by Duges, a special function which the rest cannot perform; and, on the contrary, a duty falls on the posterior segmental units of the system which the anterior cannot discharge. It is consequently evident that neither of the moieties into which the body is resolved during the crisis of the reproductive season can be organically perfect. Such fragmentary organism is wanting in elements paramountly essential to individuality.

(640). These Annelids are annuals: the term of existence is completed when the organic cycle is once accomplished. They are born during the latter months of one summer, and survive the winter, attain to the maturity of growth, reproduce the species, and die by the spontaneous subdivision of the body into fragments on the arrival of the same season of the succeeding year. This brief round comprehends the history of each individual. Since these worms are monoecious, each shares the common fate. Each contributes by its own death to the multiplication of the species: the species being multiplied, the ends of its own existence are accomplished.

(641). For some time before the fissure of the body occurs, the process of the maturation of the ova is proceeding. Arrived at the matured phase, they escape from the ovarian system into the free space of the peritoneal cavity, wherein they sojourn until the next phase of their growth has been attained. It is during the period marked by the presence of true ova in the chamber of the peritoneum, floating in the contained fluid, that the division of the body of the parent animal takes place. In each fragment is therefore incubated a considerable uumber of ova. Filled still by the fluid of the peritoneal cavity, each fragment becomes subservient to the end of hatching the young. It resists decomposition only during the period required for the accomplishment of this purpose. When the ova are committed to the sand, the fragment rapidly disappears by putrefaction. The fissure of the body, thus interpreted, becomes the last act of the parental worm, since the portions into which the body is subdivided by fissure never take food. With the fissure the necessity for food terminates.

If, on the contrary, the division of the body were the first step of a real reproductive operation, characterized by the superaddition of new segments to the body, each fragment should grow voracious and consume extra supplies of nourishment in order to provide the necessary pabulum for the reparation of mutilated parts. As this is not the fact, the inference is clear that the division of the body is not the prelude to a series of reconstructive operations by which parts are made "wholes "or mutilations repaired.

(642). Dorsibranchiata

In the second order of the Annelidans the respiratory apparatus consists of nuraerous vascular tufts, a pair of which are appended to the outer surface of every ring of the body, or, in some cases, only to a few of them. The organs of locomotion, which are attached to each segment, assume various forms, but are generally composed of short moveable spines, or packets of retractile bristles, usually destined to perform the office of oars. In the annexed figure (fig. 118,1), which represents Laodicea antennata, the general form of these animals is well seen, as is the most usual arrangement of the branchial tufts and locomotive setae. In fig. 118,2, showing an imaginary transverse section of one of the segments, the relative positions of the oars (c, d, e) and of the branchial appendages (b) are likewise indicated.

Laoclicoa antennata.

Fig. 118. Laoclicoa antennata.

(643). But the organs of respiration in the Dorsibranchiate Anne-lidans are not always arborescent; on the contrary, they are not unfre-quently spread out into thin membranous lamellae, or resemble fleshy crests or vascular tubercles: still, whatever their form, their office is the same; and the vessels spread over them presenting an extensive surface with which the water is brought in contact, the blood is oxygenated as it passes through them.