A Contribut. to the Embryol., Life-history, and Classificat. of the Dicyemids. 17 



notches fovnied by the retreating lateral angles, the dorsal is occupied 

 by two ectodermal cells , the ventral mainly by one. The dìameter of 

 the cervical collar is fnlly equal to that of the calotte , and its two cells 

 bave about the same length as the other ectodermal cells of the body. 

 The fifth and sixth cells, counting froni the hind end of the body, are 

 more granular than the others, and represent the two characteristic ver- 

 ruciform cells of later stages. The cilia of the calotte are more thickly 

 set and shorter than those of the other ectodermal cells. The two ca- 

 lottes just described (figs. 1 and 93) show about the range of variability 

 in these early stages , and their diflferences recur between the embryo 

 and the parent in fig. 1. From these forms, figures 2 and 3, taken from 

 two Rhombogens of equal length (1.20 mm), dififer in several note- 

 worthy respects. The diameter of the head is much greater than that 

 of the parapolar collar , and the metapolars flare a little at their hind 

 margin. In fig. 2, seen from the ventral face, the dorsal metapolars 

 appear to he a little larger than the ventral ; but most of the oblique 

 calottes agree with fig. 3 (seen from right side) in having the ventral 

 metapolars decidedly larger than the dorsal. The same inequality holds 

 true, with less constancy, between the ventral and dorsal propolars 

 (figs. 4, 5, 6, 9). The difference in size between the propolars and the 

 metapolars is most frequently in favor of the latter. 



One of the most uncertain and variable features of the adult calotte, 

 is its degree of obliquity , which can be said, only in a most general 

 way, and by allowing a very liberalmargin for exceptions, to increase 

 with age. 



A dorsal and a profile view of two strongly plagiotropal calottes, 

 from individuals of the same length (3.50 mm), are given in figs. 5 

 and 9. In both, the four propolars bave a ventral position; while the 

 ventral metapolars are much elongated , and so curved that one end is 

 ventral, the other dorsal, and the middle portion lateral. In bending 

 towards the dorsal side, they are at the same time prolonged forward, 

 and thus describe a curve of double curvature. This peculiarity repeats 

 itself pretty constantly in the most oblique calottes, and helps to explain 

 how the obliquity arises. 



The forms of this heteromorphous calotte , seen in Piate 1 , first as 

 hemispherical (fig. 1), then pyramidal (fig. 2), galeate (fig. 3), globose 

 (fig. 7), clypeate (figs.5and9), campanulate (fig. 6), etc., although far 

 from being exhaustive , are at least sufficient to convey some notion of 

 its Protean character , and to illustrate some of the principal lines of 

 form- Variation. They were ali taken from well developed and well 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. IV. 2 



