790 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The existence of a circulatory system is denied, apart from the 

 water- vessels and the general lacunae of the hody ; albeit that these 

 lacuna3 are so disposed between the various internal organs, or these 

 and the integument, as to present a certain definiteness in their 

 arrangement. The true madreporic tube is figured with the pyriform 

 gland (hitherto mistaken for a heart) placed beside it, the two being 

 enclosed in a common investment strengthened with calcareous pieces 

 and constituting the sand-canal of authors. The circular water- 

 vessel of the larval brittle-star is closed from the first. Our author 

 does not fully explain how it comes to surround the gullet. He 

 simply says that the aquiferous system encroaches upon the digestive 

 tube. Probably it slips over the rudimentary gut soon after the dis- 

 appearance of the anus. The tubular ring, at an early period, sends 

 forth five rays, each of which again gives off five cseca. Of these 

 cseca the central one becomes the longitudinal ambulacral vessel of 

 the arm ; the adjoining casca supply the first pair of tentacles ; the 

 two outer caBca represent the superior pair of buccal tentacles, whose 

 common trunk elongates and produces the second pair. At first all 

 five caeca have their points outwards ; at a later stage the buccal pair 

 turn in to face the mouth. The young Polian vesicle at first, also, 

 grows from the circumference of the ring towards its centre ; subse- 

 quently this direction is reversed. Not all brittle-stars have Polian 

 vesicles. They are absent in Ophiothrix versicolor, which is thus 

 distinguished from the common 0. rosula, as likewise by its less 

 convex arms and more variable tints. 



While in certain Ophiuroids, e. g. Ophioglypha, the genital organs 

 are appended to the respiratory pouches, as Ludwig has described 

 them, they are in others more or less distinct. They are so far 

 independent in Ophiocoma nigra that, when this species is dissected 

 with its dorsal aspect upwards (in its natural position) the respiratory 

 sacs must be removed before the genitalia can show themselves. In 

 Ophiothrix each cluster of genital " glands " is replaced by a single 

 organ. 



The development of the Ophiuroids is described in the case of 

 two species, OphiotJirix versicolor and Amphiura squamata. The 

 first represents that section of the group in which there is an early 

 oviposition and metamorphosis of the young ; while Amphiura, further 

 exceptional in being hermaphrodite, is viviparous. Nevertheless the 

 organogeny of these two brittle-stars presents many more points of 

 agreement than of difference. The resemblance of the free larva to 

 the pluteus of the Echini, on which so much stress has been laid, is 

 to be regarded as more superficial than real, and comparatively simple 

 larval forms may occur beside those whose very striking transitory 

 appendages render them rather abnormal than otherwise. The 

 researches of Metschnikoff, the only observer since Muller who has 

 contributed much to our knowledge of this subject, should be com- 

 pared with those of our author. In many features the development 

 of the Ophiuroids essentially approximates to that of the true star- 

 fishes, as first described with adequate fullness in the beautiful memoir 

 for which we are indebted to the younger Agassiz. 



On three important topics, demanding renewed inquiry, the 



