i895. RESULTS OF "CHALLENGER'' EXPEDITION. 47 



conditions of comparative isolation in abyssal depths, much light has 

 been thrown upon the range and character of the morphological 

 plasticity of many genera, as well as upon the probable archaic 

 characters of a number of forms previously known only from shallow 

 water. 



Several novel points of structure are met with among deep- 

 dwelling Asterids ; but mention can only be made here of two or 

 three. For instance, the Porcellanasteridae, in addition to their 

 archaic form, are remarkable for the presence of " cribriform organs," 

 — peculiar structures associated with the marginal plates, which 

 probably serve as percolators (PI. x.. Fig. 3). Many of the members 

 of this family possess a more or less elongate, epiproctal, tubular pro- 

 longation; and "segmental pits and papillae "- — organs whose functions 

 are as yet unknown — occur in some forms. 



The Pterasterid^, which are now found to possess a world-wide 

 distribution in deep water, are remarkable for the dorsal nidamental 

 chamber in which the young are kept for a period, as in a marsupium 

 {PI. X., Fig. i). Direct development is thus fostered, and the inter- 

 vention of a locomotory pseudembryo avoided. A number of other 

 Asterids belonging to different families have been discovered, in which 

 the same result is attained by different means, and not only in star- 

 fishes but in other groups of the Echinodermata, e.g., Holothurioidea, 

 Echinoidea (see Heiniaster cavernosus, PI. ix.. Fig. 4), Ophiuroidea. 

 The prevalence of forms in southern latitudes presenting this mode of 

 development has been noticed as remarkable ; it is probable, however, 

 that it will be found on further research to be more widely distributed 

 than has hitherto been suspected. 



The abyssal Asterid-fauna brought to light by the " Challenger " 

 is not the only interesting part of the collection. Many valuable 

 additions have also been made to the fauna of the Littoral and 

 Continental zones ; and some of these increase our knowledge of the 

 relationship of previously known deep-water forms, and add greatly 

 to the solidarity of the group. Under this category may be mentioned 

 the new genus, Pholidaster, which is perhaps the shallow-water 

 representative of Zoroaster ; and the interesting Peribolaster, which in 

 like manner recalls, and is structurally related to, the abyssal 

 Korethvaslcr. Pseudarchaster is an interesting annectant genus between 

 the Archasteridae and Pentagonasteridae ; and not less important are 

 the genera Leptogonaster, Tarsastev, Rhipidaster, Perknaster, and a large 

 number of specific forms belonging to genera previously known. 



It is needless to say that the knowledge of the geographical 

 distribution of types has been largely extended. 



W. Percy Sladen. 



No better account of the Ophiuroidea collected by the 

 " Challenger " can be desired than that furnished to the Narrative by 

 Mr. Theodore Lyman, after the completion of his Report : — 



