and its supposed Parasite. 223 



Krohn has succeeded in keeping alive youug Holothurids 

 with calcareous wheels, taken in spring at Messina, until they 

 lost the rows of cilia and crept in a perfectly worm-like manner, 

 "like Synapta" (Muller's Archiv, 1853, p. 319). In one 

 individual, moreover, Krohn saw the number of the still simple 

 tentacles, which is originally five, increase to eight by the 

 simultaneous growth of three new ones. In the discovery of 

 this later stage, Krohn also thought he found a support for the 

 opinion of J. Miiller already referred to, namely, that the small 

 Holothuria and Auricularia with calcareous wheels belong to the 

 genus Chirodota [Synapta? — Ed.]. 



Synapta digit at a, as it occurs in the Bay of Muggia,near Trieste, 

 reproduces in the spring (Leydig, in Muller's Archiv, 1852, 

 pp. 507, 516), and, as I ascertained, only once in the year. I 

 have observed that the appearance in great numbers of the Auri- 

 cularia with calcareous wheels in the Bay of Muggia, where 

 Synapta digitata lives in abundance on the sea-bottom, coincides 

 exactly with the time at which the distended genital sacs of the 

 Synapta contain mature ova with a germinal spot, and zoospermia, 

 and that shortly afterwards they are all found empty. 



I have also succeeded in tracing the development of the larva?, 

 pupae, and young Holothurids (which are at first easily caught, 

 but afterwards captured" with gradually increasing difficulty) up 

 to the point at which they live, in the form of small transparent 

 worms of about 8 mill, in length, in the fine mud of the sea- 

 bottom, and have gradually acquired all the anatomical and 

 zoological characters of Synapta digitata. In the last observed 

 and furthest advanced animals the number of tentacles, by a 

 further pushing out of four new ones, had become twelve, the 

 number occurring in the mature Synapta. The originally 

 simple, conical tentacles had acquired the specific form of those of 

 Synapta digitata ; they terminated in five little feelers, arranged 

 like fingers, of which the middle one was short and curved out- 

 wards. They had the suckers on the inside of the base, and 

 performed movements from without inwards, accompanied by 

 alternate extension and retraction. The skin, which was at first 

 unarmed, had become beset all over with peculiar calcareous 

 structures ; these were little anchors, each attached moveably, by 

 means of a knob, to a perforated calcareous plate. At the same 

 time, however, the calcareous wheels present in the larva were 

 still retained at the posterior extremity of the body : but their 

 number had not increased. 



From this it appears that the Auricularia with calcareous wheels 

 is the larva of Synapta digitata, Mull., and that the Synapta has 

 a young stage in which it has already attained its definitive form, 

 and possesses the anchors in its skin, but still bears the wheels of 

 the larva at its posterior extremity near the anus. 



