320 ASAJIRO OKA AND ARTHUR WILLEY. 
the Didemnide. The spicules form a distinct layer, and 
are fairly numerous. Their form is very various. A few 
varieties are shown in fig. 6, but the crenate form is perhaps 
the commonest. 
They occur in special aggregations round the branchial 
apertures of the Ascidiozooids (fig. 5). They are seen to best 
advantage in sections which have been mounted unstained.! 
At the surface the test consists to a very limited depth (fig. 
3) almost entirely of large bladder-cells, which are usually 
rendered polygonal by mutual pressure, but when they contain 
crystals they are invariably perfectly round. Below the layer 
of bladder-cells the fusiform cells occur, and the bulk of the 
crystals are contained in a zone about equal in thickness to 
the layer of bladder-cells, but mainly scattered among the 
fusiform-cells. They occur more rarely in the bladder cells at 
the very surface. 
But wherever the crystals—or, as they are usually called, cal- 
careous spicules—occur, they are always found in large round 
bladder-cells ; and, as Giard pointed out (p. 508), they are 
formed round the nucleus of the bladder-cell. 
Lower down in the test the pigment concretions begin, and 
they seem also to be produced in similar large round cells of 
the test. 
6. Systematic Position. 
According to the classification of von Drasche (6), the 
Didemnide consist of two genera, namely, Didemnum and 
Leptoclinum,’ the former having three and the latter four rows 
of branchial stigmata.> Leptoclinum, again, he splits up into 
1 It may just be noted that sections so mounted in benzole Canada 
balsam were of no use for observing the crystals, as the latter were very soon 
destroyed, probably through the influence of the impurities which frequently 
occur in benzole. ' 
2°Herdman, however, with good reason insists on maintaining the Huce- 
lium hospitiolum of Savigny, which has six rows of branchial stigmata. — 
* In von Drasche’s scheme of classification the numbers are accidentally 
transposed. In the description of genera and species they are given cor- 
rectly, 
