anatomy, embryology, physiology, biology. S 'pong. 17 
mode of colony formation, and probably go back to the Carboniferous. 
The spherical or compressed barrel-shaped individuals are arranged one 
on the other, like a string of pearls, so that elongated upright stems arise, 
characterized externally by constrictions, corresponding to which the 
internal thin- walled cavity of the whole stem is divided by partitions into 
segments or chambers, lying one over the other. By lateral budding 
larger colonies of the second order are formed. Between the stems, or 
colonies of the first order, remain systems of spaces, which are partly 
hollow, partly filled with tissue, composed of anastomosing fibres. The 
so-called partitions are the arched-in lateral walls of the individuals ; 
each has a terminal cover, which for a time formed the upper termina- 
tion of the stem, till a new chamber was formed over it. The partitions 
have large central openings, which have functioned as oscula, one after the 
other. The edges of these openings are turned upwards or downwards, 
or both ways, and the collars thus formed grow from one to the other 
partition, so as to form an axial tube traversing the whole stem, a true 
oscular canal, which is in communication by fenestrm, arranged in whorl, 
with the ring-shaped chambers or paragasters of the single segments. 
Both partitions and lateral walls are perforated by numerous fine radial 
simple canals, the radial tubes or chambers. 
A type of the group is Barroisia , ( Verticilliles , Tremacystia) anasto- 
nioscins , &c., and Thalamopora cribrosa , Goldf. In Barroisia, the skeleton 
consists mainly of triradiates without order, as in the recent Anamixilla. 
No gastral skeleton was observed, but there was a distinct dermal skeleton 
formed of fine pin-shaped spicules, with the heads directed outwards. 
They are arranged in bunches of a hollow tubular shape, forming the 
continuation of the skeleton of a radial tube, the outer extremity of 
which they leave uncovered. The cap forming the extremity of the 
stem consists, like the wall, of an inner layer of triradiates, and an outer 
one of pin-shaped spicules. But when a new segment is formed, and the 
cap becomes a partition, it becomes three-layered, since the gastrally- 
placed skeletal part of the lateral wall of the new segment grows over, 
and covers the floor of the segment. The oscular tube is formed only 
of triradiates, without “ pins.” Also the intermediate tissue formed of 
anastomosing fibres appears to consist solely of triradiates. Thus the 
Polysteganinai have the same structure as modern Sycons, and, since both 
in the former and the latter spongin is certainly absent, it may be in- 
ferred that it is absent in other Pharetrones also. 
For the Anatomy and Histology of Spongillidce, see Weltner. 
b. Embryology and Asexual Development. 
Delage (1, 2) finds that in the larva of Spongilla there is, under the 
ciliated cells, a discontinuous layer of large rounded cells, which travel to 
the exterior, and become the ectoderm after fixation. The difference 
between Spongilla and E sperella is that in the former the true ectoderm 
is entirely internal. The central nucleus of the larva is formed of large 
