764 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Synaptidae of the Mediterranean.* — Dr. E. Semon has investigated not 

 only the well-known Synapta digitata and S. inhserens, but also the S. Mspida 

 of Heller, and the first representative of Chirodota vrhich has been found 

 in the Mediterranean. A fresh systematic account of (S'. Mspida is given. 

 Chirodota venusta sp. n. is only found in the thick roots of Posidonia 

 caulini ; it differs from the Synaptae in that the five radial nerve-trunks, 

 after leaving the nerve-ring, pass out above and not through the calcareous 

 ring ; this new species appears to be near C. dunedinensis Parker. 



;S^. digitata and S. Mspida live on and not in the sand, and in their 

 external appearance they mimic well the ground on which they live ; in 

 their tentacles quite a number of animal and vegetative functions are 

 united; respiration is aided by the extraordinary lively circulation which 

 goes on within their cavities ; they serve as organs of attachment, and 

 either draw their body towards the affixed object, or, if that be small, they 

 draw it towards themselves, thus subserving locomotor or prehensile func- 

 tions. They act also as digging organs, pushing aside the sand and so 

 allowing of the intrusion of the anterior end ; by seizing sand-grains and 

 swallowing them they get the minute organisms which are found in the 

 sand. Together with the tactile papillae in the skin, the tips of the tentacles 

 serve as organs of touch. 



With regard to the development, minute structure, and morphology of 

 the calcareous spicules, what is true of Ilolothurians appears to hold also 

 for other classes of Echinodermata. The earliest stages were studied in 

 larvae of Strongylocentrotus lividus ; the first deposit of calcareous matter 

 was seen in the formative cells, where a calcareous granule of indefinite 

 form, but with a tendency to a tetrahedron with compressed sides, could be 

 made out ; this tetraxial object is developed in the interior of the cell. As 

 it grows one axis lags behind and a regular triaxial spicule arises, and 

 these lie not in but by the cell, a thin homogeneous investment surrounding 

 the spicule. With some difficulty it is possible to prove that the organic 

 investment is retained by the spicule of the adult. This is best done by 

 slowly dissolving the chalk by acids, and observing the process under a 

 high power. In large spicules it is quite easy to detect the central canal, 

 and it is almost certain that there is an organic axial substance in all calca- 

 reous bodies of Echinoderms. This axial substance does not appear to be 

 a compact mass, but to consist of a fine plexus, the filaments of which are 

 strongest in the centre. The calcareous bodies appear to consist of alter- 

 nately arranged and concentric layers of calcareous and organic substances. 

 The latter are much thinner than the former, and decrease in thickness 

 from the centre to the periphery. 



The author insists on the primitively tetrahedral condition of the 

 spicules, and points out that in some — as the wheels of Auricularise and 

 Holothurians — the tetraxial arrangement persists. A regular triaxial form 

 is one in which the rays are set at an angle of 120° ; and it is at this 

 angle that, without exception, all further divisions and branches are made ; 

 the further divisions made thus regularly must result in a regular network, 

 and it is clear that the intervening spaces between the bars must be 

 regularly hexagonal. All the complicated calcareous structures such as 

 we find in the firm skeletal parts of Crinoids, Asteroids, and Ophiuroids, 

 as well as the plates and calcareous rings of Holothurians, all consist onto- 

 genetically of a network perforated by regular hexagons. 



The calcareous bodies which remain tetraxial are morphologically the 

 most interesting; in the spines of Echinoderms the fourth axis is the 



* MT. Zool, Stat. Neapel, vii. (1887) pp. 272-300 (2 pis.). 



