PYROSOMA THE FOUR PRIMARY ASCIDIOZOOIDS. 



413 



Kowalevsky that the small Pyrosoma colonics contained only male 

 zooids, females being found in the large colonies. 



Anteriorly the supra-intestinal blood-sinus surrounds the rudiment 

 of the central nervous system. Further back it is continued into the 

 furrow which seems to be formed by the infolding of the endostyle- 

 rudiment. Since, between the nervous system and the endostyle- 

 rudiment the branchial aperture occurs, the stream of blood would 

 be interrupted but for a peculiar adaptation for conducting it further, 

 the upper wall of the intestine becoming folded in at this point 

 somewhat in the manner of a typhlosole (Figs. 204, 205, dl>). This 

 fold within which the blood now runs becomes completely separated 

 from the dorsal wall of 

 the intestine, and then 

 forms a tube running 

 freely through the intes- 

 tine from the neural 

 rudiment (Fig. 205, n) 

 to the beginning of the 

 endostyle - furrow (en). 

 This peculiar structure, 

 which has been called 

 by Salensky the 'pha- 

 ryngeal blood-sinus, and 

 by Huxley the diax>ha- 

 ryngeal band, may be 

 compared with the gill 

 of the Thaliacea with 

 which Salensky even homologises it, although topographically it is 

 clearly only an analogous structure. The diapharyngeal band is 

 merely a provisional adaptation which atrophies in the further 

 course of development. 



Two structures, the significance of which is obscure, the elongate aud the 

 lenticular cell-masses (Keferstein and Ehlers) are to be traced back to the 

 mesoderm. The lenticular masses are paired and symmetrical accumulations 

 of cells lying at the entrance to the branchial sac between the wall of the 

 peribranchial cavities and the entoderm (Figs. 196, /, and 106, liii. p). 

 Salensky considers that they are to be derived from the kalymmocytes 

 (inner follicle-cells). They are said to be phosphorescent organs. The 

 elongate masses, which lie on the neural side "near the gills in the blood- 

 sinus," form later from an unpaired accumulation of mesoderm-cells which 

 originally lies below the so-called languets of the entoderm (Figs. 196, d. 

 106, dm). 



Fis. 205. — Longitudinal section through an Ascidio- 

 zooid of Pyrosoma (after Salensky). db, diapha- 

 ryngeal band ; el, elaeoblast ; en, endostyle ; i, 

 branchial aperture ; m, stomach ; ms, sphincter 

 muscle ; >i, rudiment of the ganglion ; oe, oesoph- 

 agus ; v, connective strand. 



