ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 387 



not be different from those of Bonellia, and according to Kietsch all 

 are artificially produced. Nor do the ventral lacuna? of Echiurus 

 seem different, their occurrence in this form only is explained by the 

 peculiar disposition of the musculature on the inferior surface of the 

 proboscis. In these lacuna?, Eeitsch never found ova, sperms, or 

 other elements characteristic of the perivisceral fluid. The lacunae 

 are not portions of the body-cavity. The musculature is the only 

 active factor in extension and retraction. 



The proboscis is remarkable for the abundant nerve-terminations, 

 and for the direct relations between epidermis and nerve-centre. 

 On the anterior surface of the horns in Bonellia, there is a special 

 organ of taste and touch. The variable histological structure of the 

 margins is discussed. The lobe discbarges sensory, prehensile, and 

 locomotor functions, and attains of course its highest development in 

 Bonellia. 



9. Segmental organs. — The genital pouches of the Echiurians are 

 homologous with the segmental organs of Annelids. Their number 

 varies from one to four pairs in the Thalassemas, in the Echiurians 

 there are always two pairs. The funnels end in twq long spiral 

 grooves, or in two unequal, extensile lips. In Thalassema gigas, 

 which has only one pair of pouches, the funnels are more regular in 

 form, and the same is true of Hamingia and Bonellia. The first has 

 usually two uteri, but exceptionally only one ; this exception is the 

 rule in Bonellia. The occurrence of a B. viridis with two uteri 

 (Lacaze) also prevents the separation of the single uterus type from 

 the former with paired pomhes, or indeed from the segmental organs 

 of Annelids. The reduction to a pair of pouches is probably asso- 

 ciated with the sexual dimorphism. If there were several pouches, 

 some ova would run a risk of not being fertilized.. The males and 

 the ova ax*e separated within the single uterus till sexual maturity is 

 attained. The eggs when expelled and fertilized are enveloped in a 

 tenacious mucus which remains, connecting the embryos and even the 

 larva?. There are thus greater possibilities for the sexes falling in 

 with one another. 



10. The ovarian duet, the ovary, and the development of the eggs. — In 

 some Echiurians (Thalassema, Echiurus, Hamingia glacial is), the primi- 

 tive ovules are modified directly into ova ; in others (some Thalassemas, 

 Hamingia arctica, and most definitely Bonellia) accessory structures in 

 the form of follicle and envelope are formed. In the first case, the 

 germinal elements arise from peritoneal cells forming the external 

 envelope of the ventral vessel. In the second case, they have, accord- 

 ing to Spcngel, the same origin, but according to Greef they arise 

 from cells within the peritoneal layer. Kietsch's results agree most 

 closely with those of Greef. 



A short description is then given of the male Bonellia, and the 

 long memoir of 200 pages closes with some notes on the development 

 and on the systematic position of the Gephyrea in "relation to the 

 higher Annelids. A classification and diagnoses of the different forms 

 are also added. The monograph is accompanied with very abundant 

 illustrations. 



