ASTEROIDEA. 395 



so excavated on their sides as to give rise by their apposition later- 

 ally to a series of pores, by means of which the ampullae communi- 

 cate with the bases of the tube-feet. Hence, the ampullae are 

 situated in the interior of the arms, superiorly to the chain of 

 ambulacral ossicles, while the radiating ambulacral vessels and 

 tube-feet are placed i?iferiorly to the same. 



In addition to these ambulacral vessels, Star-fishes possess a system 

 of respiratory organs which are known as the dermal branchiae or 

 "papulae." These are delicate caecal processes of the integument, 

 the cavities of which are in direct relationship with the general 

 body-cavity. In one order of recent Star-fishes (Phanerogonia) they 

 are restricted to the dorsal surface, while in the Cryptogonia they are 

 distributed over the whole body. 



The generative organs of the Asteroidea are situated within the 

 "arms," above the chain of ambulacral ossicles, and they discharge 

 their products by means of minute sieve-like openings in the angles 

 between the arms, the size of these apertures being too small to 

 admit of their recognition in a fossil condition. 



The living Asteroids have a wide range in space, being princi- 

 pally shallow-water forms, but extending from the littoral zone to 

 great depths in the sea. As regards their distribution in time, the 

 Star-fishes are a group of great antiquity, the earliest members of 

 the order appearing in the Ordovician (Upper Cambrian ?) strata. 

 Most of the Palaeozoic types are peculiar, but the recent genus 

 Astropecten is stated to occur in rocks as old as the Devonian. 



As regards their classification, the Asteroids may be divided into 

 two primary groups, or sub-classes, according to the arrangement of 

 the ambulacral ossicles. In one great group (Euasteroidea), the two 

 rows of ambulacral ossicles which roof over each ambulacral furrow 

 are placed opposite to each other. The forms of this group are 

 not unrepresented in the Palaeozoic rocks, but they are principally 

 characteristic of the Secondary and Tertiary deposits, and are the 

 only types now in existence. On the other hand, most of the 

 Palaeozoic Star-fishes belong to the series of the Encrinasterice, in 

 which the two rows of ambulacral ossicles in each arm are so placed 

 as to alternate with one another. No form belonging to this group 

 of Star-fishes appears to have survived the Palaeozoic period. Owing 

 to their rare occurrence as fossils, and their generally imperfect state 

 of preservation, Star-fishes are not of special interest to the palaeon- 

 tological student, and it will be sufficient here to briefly indicate 

 the chief characters of some of the more important of the known 

 fossil forms. 



The section of the Encrinasterice, in which the ambulacral ossicles 

 are alternately placed, is, as above remarked, exclusively Palaeozoic, 

 and the widely distributed genus Palceaster may be taken as its 



