ISOMETRA VIVIPARA. 41 



upper side of the stomach (plate xvi, figures 5 and 6). The notch in the outer 

 side of this ccelom seen in plate xvi, figure 9, is part of the chambered organ, 

 its peculiar shape being due to the fact that the section has been obliquely 

 directed. It seems unnecessary to give a detailed description of the shape of 

 this ccelomic cavity, which may be gathered from the figures in plates xvi 

 and xvii. The axial organ has begun to develop as a thickening of the ccelomic 

 epithelium in the notch at the vertical mesentery (plate xvn, figure 11). 



The entoderm still remains in the same histological condition as in the 

 preceding stage, the lumen being indistinctly limited or entirely obliterated 

 on account of the inwandering yolk-cells. In plate xvn, figure 6, the epi- 

 thelium is in places more distinctly limited against the lumen, while in other 

 places yolk-cells appear to be in the act of wandering into the lumen ; opposite 

 the hydroccel the entodermic epithelium is quite thin, almost endothelial; 

 this is, however, an individual variation. The shape of the stomach is no 

 longer quite simple, on account of the notches from the aboral coelom, being 

 now somewhat lobed (plate xvi, figure 6) ; one of the lobes probably is des- 

 tined to form the intestine, but I can not ascertain this with full certainty. 

 Yolk globules, some being very large, are still seen in the entoderm as well 

 as in the mesoderm (plate xvi, figures 3 to 5; plate xvm, figure 8). 



5. THE PENTACRINOID STAGE. 



The fact that the vibratile bands of the larva are well developed proves 

 that the free-swimming stage has not been entirely done away with, as is 

 the opinion of K. A. Andersson. On the other hand, the larva must, on 

 account of the strongly developed skeleton, be rather heavy, so that it must 

 be a poor swimmer and probably sinks almost straight down, where it meets 

 the point of the upturned cirri and attaches itself. The fact that the Penta- 

 crinoids are not found attached to all the upturned cirri, but generally only 

 to two or three of them, would seem to indicate a slight amount of swimming, 

 as one would otherwise expect them to be attached to the cirrus just below 

 the place whence they have come. 



Dr. K. A. Andersson (op. cit., p. 7) states as an advantage acquired by 

 this supposed giving-up of the free-swimming stage that the larvse "laufen 

 nicht Gefahr, in zu tiefes Wasser oder in andere Teile des Meeres hinausge- 

 fiihrt zu werden, wo sie auf einen ungeeigneten Boden niedersinken und zu 

 Grunde gehen konnten." While this may be true, still the larvae are not 

 free of danger during their short passage from the marsupium to the cirrus- 

 tip; on the contrary, they there incur the danger of being eaten by their 

 elder brothers and sisters. As I have stated in my report on the Crinoids 

 of the Swedish Antarctic Expedition 27 (p. 15), I have found quite a large 

 percentage of the Pentacrinoids to contain in their stomach the half-digested 



27 Wissensch. Ergebnisse d. schwedishen Siidpolar-Expedition 1901-1903, Bd. vi, 8, 1917. 



