246 SUMMARY OF CURRENT RESEARCHES RELATING TO 



genital bud of the young urchin is enveloped by a membrane which is 

 continuous with that which envelopes the ovoid organ, but it seems to be 

 always separated fiom it — it can only be said that the primitive genital bud 

 appears as a simple dependent of the mesenteric plate which surrounds the 

 ovoid gland and the aquiferous tube. 



Distribution of Sea-TJrchins.* — Dr. W. Haacke communicates some 

 interesting notes on the habits and distribution of sea-urchins considered 

 in relation to their past history. 



(1) In the iirst place he notes the characteristics and occurrence of two 

 litoral Australian species, Amblypneustes ovum and A. formosum, which occur 

 among the sea-grass and tangle banks. The former is found exclu- 

 sively among the sea-grass ; its colour corresponds to the greenish-yellow 

 light of such a habitat, and its form is well adapted to its habit of climb- 

 ing up and down on the sea-weed. 



(2) Haacke emphasizes the necessity of caution and criticism in regard 

 to what is often said in regard to the persistence of fossil forms in the great 

 depths. Tlie phylogenetically older regular sea-urchins are better repre- 

 sented, as Neumayr has shown, near the coast than in the deep sea. 

 Uniformity of external conditions does not necessarily imply an unchanged 

 persistence of ancestral environment, admitting of the persistence of primi- 

 tive forms. 



(3) Deep-sea fossils of previous epochs are unknown to us, and deep-sea 

 forms must be compared with deep-sea forms. The preservation of litoral 

 forms is very scanty, the conditions were not favourable, and the dead sea- 

 urchins buoyed up by the gases of putrescence would then, as now, float away, 

 and be broken up on the shore. The fossil remains both of the litoral and 

 the uj)per continental areas are so scanty that a comparison of living and 

 extinct forms becomes very hazardous. In the lower continental zone the 

 preservation of fossil remains is more complete, and the recent extension of 

 our knowledge of living forms has naturally led to the discovery of " living 

 fossils." " The partial persistence of external conditions chai'acteristic of 

 earlier epochs has indeed favoured the survival of ancient forms, and such 

 a persistence is not to be found exclusively in the deepest depths of the 

 ocean, but for terrestrial animals on land, for fresh-water animals in their 

 medium, for litoral animals near the shore, for ' continental ' animals in 

 their own zone, and only for true deep-sea forms in the abyssal region. A 

 consideration of variations in habitat, mode of life, and forms must be 

 associated with phylogenetic investigation. When this is done there will 

 be no more marvel that the exploration of the deep sea has not revealed 

 more 'living fossils.' It is younger than the shallow water, and in virtue 

 of its peculiarity has allowed many of the old forms (which wandered into 

 it from the latter) to die out, while others it has greatly modified." 



Formation of Genital Organs and Appendages of the Ovoid Gland in 



Asterids-t — M. L. Cuenot thinks that it is impossible to interpret the 

 vascular system of Asterids without having recourse to the development of 

 the genital organs. In a young star-fish in which the gonad has not begun 

 to be formed there is on the aboral and internal surface of the test a dorsal 

 blood-vascular ring, which, at each interradius, gives off two csecal vessels 

 which are directed towards the extremity of the arm. In one interradius 

 this ring communicates with the large sinus which incloses the ovoid gland 

 and the sand-canal. At this time the gland is ovoid, but a little later it is 

 prolonged into two buds which go to the right and left j these extend 



* Biol. Centralbl., vi. (1887) pp. 641-7. 

 t Comptes Rendus, civ. (1887) pp. 88-90. 



