210 
MOLLUSCOIDA. 
Diplosomatidj]. 
Aatellium 8 pongiforme{Q(i 2 LvdL). Development of the larvae, which are 
at first free swimmers and afterwards fix themselves, described by Giard, 
0. R. Ixxxi. p. 1214 ; R. Z. (3) iii. p. Ixx. 
Pyrosomata. 
The development of Fyrosoma has been studied by A. Kowalewsky, 
who describes the partial segmentation of ^}ie egg, and the formation of 
the embryonal disk and its two layers, in which at a very early stage the 
first traces of the later perithoracic tubes appear. The embryo is simple 
in this stage, representing Huxley’s “ cyathozoid ; ” but in the subse- 
quent stage, when the hinder end of the embryo separates and elevates 
itself from the nutritive base, four transverse constrictions appear, by 
which the embryo is divided into the anterior nutrix or cyathozoid and 
four ascidioids, following each other in their longitudinal axis. The orifices 
of the perithoracic tubes unite and form a funnel-shaped terminal orifice 
in the cyathoid, opposite the first ascidioid, and answering morphologi- 
cally to a cloak and not to a mouth. As to the further development, 
the author has observed, that the male sexual elements make their 
appearance in young colonies of not more than an inch in length, long 
before the eggs. Arch. Mikr, Anat. xi. pp. 697-635, pis. xxxvii.-xli. 
Pyrosoma, Brilliant spectacle offered by it at the Kermadek Islands, 
Pacific ; Willemoes-Suhm, Z. wiss. Zool. xxv. pt. 2, p. xxxvii. 
Salpj;. 
Todaro has published in full his researches on the embryology of 
Salpa pinnata [Zool. Rec. xi. p. 195]. First, he states that although both 
sexes are united in the same individual, fecundation by another is needed 
for the production of eggs, the elements of which, as soon as the “blasto- 
dermic circle appears, divide themselves into two portions, one develop- 
ing itself immediately into a solitary Salpa, the other emigrating and 
giving origin to a heap of cells, situated at first between the skin and the 
outwards-bent part of the branchial sac of the solitary embryo, and after- 
wards included in the middle layer of the skin. This organ, no true ovary, 
produces by gemmation the aggregate individuals, which are to be re- 
garded not as children, but as younger brothers of the solitary individual, 
which represents the elder brother, unmarried, fostering the younger ones. 
This view is intermediate between that proposed by Chamisso, and now 
almost generally adopted, that a true alternating generation exists in the 
Salpce, and the other defended by Eschricht, that the same individual pro- 
duces at one time solitary and at another aggregate or chained Salpce. In 
both forms, the three usual embryological strata are observed, the middle 
or mesoderm giving origin to the circulatory organs, and the outer or 
ectoderm to the skin and nervous system ; but this ectoderm is simple 
in the aggregate, and subdivided into two layers in the solitary form. 
The intestinal cavity is formed by invagination of the ectoderm in the 
solitary form ; in the aggregate, by lateral folding of the whole blasto- 
