16 AMPHINEURA. 



The dorsal and lateral parts of the body bear spicules ; on the back,, 

 these are inclined towards one another and form a projecting ridge 

 in the dorsal median line, elsewhere (especially at the sides) the 

 spines become flattened and imbricated.* A groove is found on 

 the ventral surface as in many other Solenogastres (Fig. 147 B, 

 Chap, xxxiii.). 



Dondersia lays its eggs singly. They are opaque and surrounded 

 by a delicate envelope which is only developed after the egg has 

 passed from the pericardium into the nephridia which serve as genital 

 ducts. According to Pruvot, the envelope is formed by the latter, 

 which no longer possess the function of excretory organs. 



The cleavage is, from the first, slightly unequal ; the egg breaks up 

 into two unequal blastomeres which, through division, yield three 

 small cleavage-spheres and one large one (cf. Lamellibranchia, Fig. 

 11 C). The next stage, one of eight cells, is reached by the biparti- 

 tion of the micromeres and the unequal division of the macromere. 

 No further micromeres are cut off from the macromere, which now 

 breaks up into two and then into four macromeres, while the seven 

 primary micromeres divide in fourteen and then into twenty-eight. 

 In this stage, the invagination begins, and the entoderm-cells dis- 

 appear within the cap formed by the more rapidly dividing ectoderm- 

 cells. The embryo now has a conical cap-like shape, the aperture of 

 the invagination occupying the whole of its lower side. At this, 

 time, the cilia appeal', a ciliated area arising at the frontal pole, i.e., 

 at the pole opposite to the blastopore (Pruvot), as well as round 

 this latter ; a ciliated ring also forms which encircles the whole body 

 about half-way up, the cilia here being specially strong. This stage 

 may be compared with the one described above in Chiton in which the 

 ciliated ring rises at the gastrula stage. 



The embryo of Dondersia soon changes its shape, the body becoming- 

 divided by circular grooves into three sections : (1) an anterior 

 section consisting of two rows of ciliated cells with a few specially 

 strong and long cilia at the apex ; (2) a middle section, carrying the 

 ciliated ring ; and (3) a very large posterior section also formed of 

 two rows of cells, at the end of which lies the aperture of invagina- 

 tion. Pruvot regards this latter as the blastopore, and points out 



* These statements, which are necessary to enable us to understand the 

 form of this animal, are taken from a short treatise by Pruvot (No. 12) on 

 some Solenogastres of the Mediterranean. The author intends to publish 

 a more detailed work on the structure and development of these forms. 

 Hubrecht (No. 5) has published a treatise on the organisation of another 

 species of Dondersia (D. fcstiva). 



