78 THE VOYAGE OF H.M.S. CHALLENGER. 
In the building up of the ccenosteum, which must be deposited, as in Millepora, by 
the ectodermal covering of the ccenosarcal canals, absorption of already formed hard 
structures must take place during the gradual increase in size of the ampulle and the 
widening of canals, which, as shown in the figures, are larger in bore in the deeper 
than in the younger superficial regions of the coenosteum. A re-deposit must also take 
place constantly, for old ampull, in the deeper parts of the ccenostea, are to be found in 
all stages of obliteration. Sometimes in some genera, a rejuvenescence of parts of the 
ccenosteum takes place ; a previously dead area becoming overgrown from its margins by 
a living lamina, which spreads over and covers it. 
PARASITES OF THE STYLASTERIDA. 
The ccenostea of nearly all Stylasteridee are liable to become much distorted in growth 
by the presence upon them of parasites of various kinds, each of which appears by the 
special kind of irritation which it offers to produce a particular form of abnormal 
crowth in the part of the ccenosteum it infests, producing thus, as it were, an animal 
gall. The commonest distortion is the reduction of the stem of a coral or branch, or 
of one side of these, into a hollow canal or deep furrow, more or less roofed over by a 
thin wall. This condition is produced by the adherence to the growing stem of an 
Aphroditacean Annelid. It has been noticed and described by Pourtalés' and Verrill, 
in Stylaster erubescens and Allopora californica. 1 have seen it in Cryptohelia, 
Stylaster, Allopora, and Errina. On Errina labiata, a parasitic filiform Nemertean 
also occurs which twines itself round the tips of the branches in many coils. The 
branches thus irritated grow out into a burr-like mass of projecting points which are 
evidently hypertrophied dactylopore prominences, and sometimes assume almost the 
appearance of the normal spines of Spinipora. 
The most interesting parasite observed was a form found in the gastric cavities of 
the gastrozooids of Pliobothrus symmetricus contained in small capsules. These capsules 
were badly preserved, but there seemed little doubt that they contained the remains 
of larvee of a Pycnogonid, so that the deep-sea Pyenogonids, which are so abundant, 
very possibly pass through their early stages in deep-sea Stylasteridee. The formation of 
a calcareous ccenosteum has not vitiated the capabilities of the Stylasterid Hydroids as 
hosts for Pycnogonid larve. The gastrozooids containing the larve were partly 
aborted. 
DIsTRIBUTION IN SPACE AND TIME OF THE STYLASTERID. 
The Stylasteridee range all over the world, and exist at all depths from shallow 
water on the coasts to great depths in the open oceans. Two species occur close at 
* Bull. Mus. Comp, Zool., Harvard, vol. vi. p. 136. 
