THE CALCAREOUS SPONGES, Iq! 
“ good species” of which, in fact, cannot be spoken of in the 
usual systematic sense. We find among them only varying 
series of forms, which do not even completely transmit their 
specific form to their nearest descendants, but by adaptation 
to subordinate, external conditions of existence, perpetually 
change. It frequently occurs here, that there arise out of 
one and the same stock different form-species, which accord- 
ing to the usual system would belong to several quite distinct 
genera; this is the case, for instance, with the remarkable 
Ascometra (Frontispiece, Fig. 10.) The entire external bodily 
form is much more pliable and protean in Calcareous Sponges 
than in the silicious sponges, which are characterized by 
possessing silicious spicula, forming a beautiful skeleton. 
Through the study of the comparative anatomy and ontogeny 
of calcareous sponges, we can recognise, with the greatest 
certainty, the common primary form of the whole group, 
namely, the sack-shaped Olynthus, whose development is 
represented in the Frontispiece (compare its explanation in 
the Appendix). Out of the Olynthus (Fig. 9 on the Frontis- 
piece), the order of the Ascones was the first to develop, out 
of which, at a later period, the two other orders of Cal- 
careous Sponges, the Leucones and Sycones, arose as diverg- 
ing branches. Within these orders, the descent of the 
individual] forms can again be followed step by step. Thus 
the Calcareous Sponges in every respect confirm the pro- 
position which [ have elsewhere maintained: that “the 
natural history of sponges forms a connected and striking 
argument in favour of Darwin.” 
The second main class or branch in the tribe of Zoophytes 
is formed by the Sea-nettles (Acalephe, or Cnide). This 
interesting group of animals, so rich in forms, is composed 
