REPORT OX THE TUXICATA. 
395 
Pyrosomzi, then, may be regarded as a highly modified form derived from the ancestral 
Didemnidge (H. in fig. 13), and much more closely allied to the ordinary Compound 
Ascidians, such as the Distomidm and the Polyclinidse, than to the other pelagic Tunicates, 
such as a colony of Salpse. 
The ancestral Didemnidae which were derived from the point IL divided into two series, 
those leading to the Didemnidm proper, and those which have given rise to the Diplosomidse. 
From the former, near the point of division, arose the side branch leading to the genus 
Euccelium, where the number of rows of stigmata in the branchial sac is greater than 
three or four, thus resembling most of the ancestral Distomidee, from which the Didem- 
nidae were derived. In the family Didemnidae the property of producing calcareous 
spicules in the test has reached its greatest development. Similar spicules (apparently 
cl 
A B C 
Fig. 14. — Diagrams showing the relations between — A, a typical Compound Asckiian', B, Caiooormus ; C, Pyrosouia. In all 
cases the colonies are represented in longitudinal section, and cl. indicates the opening of the common cloacal cavity. 
in aU cases formed by modified test cells, see j). 271) are found also in the Diplosomidae 
and in the Ccelocormidae, and a somewhat dihrerent form of spicule occurs in the genus 
Cystodytes, consequently it is possible that the tendency towards t^e formation of 
calcareous deposits by the test cells was developed as far back as the ancestors of the 
genera Distoma and Cystodytes (see fig. 13), and if so, then the tendency has been 
repressed in the species of Distoma. 
In the ancestral Didemnidae the male reproductive organs became concentrated to 
form a single large ovate testis around which the long vas deferens was coiled spirally (see 
p. 254). The genus Didemnum is rather less modified than Leptoclinum, which forms 
the termination of this branch. In Leptoclinum the colony has become greatly flattened 
from above downwards so as to form in most cases a mere incrusting film in which the 
test is usually densely crowded with calcareous spicules. One result of this flattening 
of the colony in the more modified Leptoclinids is that remarkable bending of the body 
