4 Tun . 
TUNICATA. 
Development. 
Herdman (4) traces the course of development by gemmation in some 
of the new ‘ Challenger’ Compound Ascidians. He finds that in Sarco- 
botrylloides , amongst the Botryllidce , buds are formed in the vascular 
prolongations, or stolons, from the parent Ascidiozooid into the common 
test. In the genus Colella he found that buds were formed in the. 
peduncle of the colony, and were afterwards carried upwards to the 
region where they appear as young Ascidiozooids. 
Brooks (1) considers that the process of gemmation in Salpa has been 
misunderstood by all previous investigators. He has recently worked it 
out in Salpa cabotti and another larger species, from Wood’s Hole, on 
the American coast, and finds that the budding is in reality a very simple 
process, directly comparable with the budding of Pyrosoma. The buds 
are produced from the stolon in a single series, but afterwards, as the 
result of crowding, become placed in two rows, the ascidiozooids of which 
have their haemal surfaces towards the middle line of the stolon and 
their neural surfaces external ; the right sides of all which lie on the left 
side of the stolon, and the left sides of all the others are towards its 
base. When first formed on the stolon in a single series, all the neural 
surfaces are proximal in their origin, and all the haemal surfaces are 
distal, while the right halves of all the bodies arise on the right half of 
the stolon, and the left halves on the left. The middle plane of symmetry 
in the Salpa stolon is identical, like the middle plane of the stolon Of 
Pyrosoma , with the middle planes of the bodies of all the Ascidiozooids. 
The final position of the body of the chain Salpce is the result of two 
secondary changes : (1) during the development and growth of the chain 
the constituent Salpa: are crowded alternately to the right and to the 
left ; and at the same time, (2) each Salpa rotates upon its long axis until 
its haemal surface becomes internal and its neural surface external. The 
stolon arises from the haemal surface of the solitary Salpa , and consists 
of an outer wall of ectoderm, an inner endodermal tube which opens into 
the branchial sac of the solitary Salpa and arises in the fold between the 
halves of its endostyle, a nerve tube, an ovary, two cloacal tubes, and 
two mesodermal tubes which give rise to the muscles. The relation 
which the solitary Salpa bears to the first chain Salpa in -the series (the 
last which it produces) is exactly the same as the relation between this 
one and the next, and therefore, according to Brooks, the egg-embryo, or 
solitary Salpa , is really a member of the chain series, and the only essential 
difference between it and the other members of the chain is its more 
rapid growth. The ascidiozooids are not formed by budding from the 
walls of the stolon, but by the direct conversion of its tissues and cavities 
into those of the Salpa. 
Geographical and Bathymetrical Distribution. 
A map is given by Herdman (4) showing the distribution of the 
Simple and Compound Ascidians obtained during the 1 Challenger’ Expe- 
dition, and tables are given showing the distribution of the families, 
genera, and species in the great ocean basins. They show that most of 
the families and genera have a very wide horizontal range. Compound 
