In Aspidosiphon fuscus both ovaries and testes have the form of mesenterial folds attached behind the ventral retractors of the pharynx on either side the nerve cord, between two bundles of circular muscles. The ova are detached, and ripen in the coelome; the sperm is formed in situ, and then detached. In Phascolosoma falcidentatus the ovaries are similar to those of Aspidosiphon, similarly placed, but attached between the longitudinal muscles. Its testes are not known. As a rule, the ova and the cells from which the sperm originates, are set free at an early period into the coelome, hence 'swimming ovaries and testes.' In the Echiuridae the glands are formed at the posterior end of the ventral vessel, and from the coelomic cells which constitute its walls. In Thalassemia and Echiurus the young ova and corresponding male cells are simply enlarged superficial coelomic cells, which are set free and develope into ova and sperm. In Bonellia each cell-mass thus liberated, consists of an envelope of delicate coelomic epithelium inclosing a mass of cells, one of which is larger and becomes ovum, whilst the rest supply it with secondary yolk.
The Gephyrean egg is inclosed in a membrane which is generally striated.
1 Sperm and ova may alike be found floating in the coelome of Sipunculus nudus, but according to Brandt, most individuals of this species contain either ova or sperm. The simultaneous presence of both products he explains as possibly due to an abnormality, unless it be supposed that the animal is hermaphrodite, and that the two products ripen as a rule at different times, but occasionally at the same time. A Gregarine, Monocystis Sipunculi, is not uncommon in the coelome of the species in question. A Gregarine occurs also in Echiurus Pallasii and Thalassema Neptuni.
The dimorphic male of Bonellia viridis has the following structure. It is a Planarian-like organism, 1-5 mm. long, pointed at each end. The hypodermis consists of a layer of ciliated cells with a delicate cuticle. The muscular layers of the body-wall are an outer circular, a spiral and an internal longitudinal layer (Spengel), with a mass of vacuolated cells lodged in a cell reticulum, and covered internally by flat coelomic epithelium, which is reflected over the intestine and vesicula seminalis. Dorso-ventral bundles of fibres placed at regular intervals and projecting into the coelome, give an appearance of septa. The coelome does not extend up to each end of the body. The nervous system consists of a peri-intestinal ring and ventral cord. The intestine ends blindly in front and behind, and is attached at both ends by muscle-fibres to the body-walls. Its own walls consist of a single layer of cells; its contents are fatty. Two simple excretory organs, the homologues of the anal caeca of the female, are situated posteriorly, one on either side, in front of the end of the intestine. Each of them consists of a tube with sparsely ciliated walls hanging in the coelome, into which it opens by a ciliated funnel. Its external pore is minute.
The spermatozoa are formed from peritoneal cells, and are differentiated from an outer layer of cells surrounding a central cell, the latter undergoing atrophy. The vesicula seminalis opens at the anterior apex of the body. Its duct is narrow, and passes inwards through the nerve-ring, when it enlarges into an elongated sac. The sac opens into the coelome by a ciliated funnel, which Spengel believes communicates with the sac itself, not terminally but laterally. The male larva clings to the under side of the prostomium of the female, and migrates thence into the mouth, where it undergoes its final changes. It then wanders into the outer chamber of the uterine pouch, which in B. minor at least is cut off by a fold from the inner portion where the ova accumulate. The males of B. viridis, and of Hamingia, are found in the same place. They are furnished with a pair of chitinoid hooks, placed anteriorly and ventrally1.
The larva of Echiurus is a typical Trochosphere. The most important features of its development are the following. There is a large prostomium, which becomes the 'proboscis' of the adult. The mouth is ventral, the anus posterior and terminal. There is a double praeoral ring of cilia, a postoral ring, and between the two a ciliated adoral zone, which is continued as a ciliated ventral furrow from mouth to anus. There is also praeanal ciliated ring. The mesoderm is segmented. Fifteen somites with a terminal piece are indicated by septa internally, by ciliated rings externally. These rings are replaced in the young Echiurus by zones of spinous tubercules, and in somites fourteen and fifteen by the circles of setae. Four zones of tubercules are formed anteriorly, in addition to the first of the series referred to. The nervous system is developed (i) from an ectodermal thickening at the apex of the prostomium, from which commissures grow down to become connected with (2) the ventral cord. The latter arises at first as segmentally-arranged thickenings of the epiblast, which subsequently fuse. The fibres of the cord are derived from the cells of the ventral furrow, which is perhaps invaginated.
There is a pair of provisional cephalic nephridia, ciliated, branched, and the terminations of the branches ending in aborted flame-cells. The anal sacs originate from the mesoblast as two tubes, opening by an internal funnel and an external pore, situated close to the anus. Their communication with the rectum in the adult is therefore secondary.
1 Spengel's male Bonellia with a pair of ventral hooks probably belonged to B. minor. Cf. p. 411 of his paper, cited below.
Thalassema mellita (Conn) has an invaginate gastrula. A ciliated band appears round the blastopore, and is replaced in the Trochosphere by a praeoral ring or row of long powerful cilia. A postoral and praeanal band are subsequently formed, and a ventral ciliated furrow. A group of stiff cilia (? sense hairs) mark the apex of the prostomium. The alimentary canal is formed entirely from the invaginated archenteron. The blastopore persists as the mouth. The mesoderm forms two ventral bands (as in Echiurus and Chaetopoda), but stellate mesodermal cells are also derived from the hypoblast, as in Holothurians, etc.