78 THE VOYAGE OF H.M.S. CHALLENGER. 



In the building up of the coenosteum, which must be deposited, as in Mittepora, 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 ampullae 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 ampullae, in the deeper parts of the coenostea, are to be found in 

 all stages of obliteration. Sometimes in some genera, a rejuvenescence of parts of the 

 coenosteum takes place ; a previously dead area becoming overgrown from its margins by 

 a living lamina, which spreads over and covers it. 



Parasites of the Stylastertd^e. 



The coenostea of nearly all Stylasteridae 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 

 growth in the part of the coenosteum 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 Pourtales 1 and Verrill, 

 in Stylaster erubescens and Allopora californica. I 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 larvae of a Pycnogonid, so that the deep-sea Pycnogonids, which are so abundant, 

 very possibly pass through their early stages in deep-sea Stylasteridae. The formation of 

 a calcareous coenosteum has not vitiated the capabilities of the Stylasterid Hydroids as 

 hosts for Pycnogonid larvae. The gastrozooids containing the larvae were partly 

 aborted. 



Distribution in Space and Time of the Stylasteridae. 



The Stylasteridae 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 



1 Bull. Mus. Comp. Zool., Harvard, vol. vi. p. 136. 



