294 CONTRIBUTIONS TO THE NATURAL HISTORY 



specks the circular tube runs undivided and unconnected with radiating tubes, there being 

 only four of these latter ; but next, again, a radiating tube reaches the middle of the space 

 between the two following eye-specks ; and so on, in such a manner that there are alter- 

 nately two eye-specks on the two sides of a radiating tube, with two eye-specks between 

 two radiating tubes, at a distance from each other of twice that from the tube to the 

 eye-speck. This singular alternation is quite a prominent feature of the genus Tiarop- 

 sis, and seems to me the more interesting, as, up to the present time, it is the only known 

 instance of the kind, and has not been noticed among the numerous otherwise similar 

 forms which occur on the British shores, and have been so minutely described by Pro- 

 fessor Forbes. Like the tentacles of Hippocrene, which are also destitute of an inter- 

 nal cavity, those of Tiaropsis elongate and shorten to a considerable extent (Plate VI. 

 Fig. 13) ; but their power of extension and contraction can by no means be compared 

 to what we see in Sarsite, which, at times, seem almost to drop their tentacles, so sud- 

 denly are they extended without any apparent action of the animal, and then draw 

 them violently in again. 



The eye-specks at the base of the tentacles are simply an accumulation of pigment- 

 cells, closely packed together without apparent order or regularity. But not so with the 

 large eye-specks. Each of them (Plate VI. Fig. 17) has within its base a large mass 

 of very black pigment-cells, a, forming a sort of nucleus to a ganglion analogous to that 

 of Hippocrene, in the formation of which the sensitive substance seems to take a con- 

 siderable part. This ganglion is crowned by a diadem-like double wreath of peculiar 

 cells, the inner row of which, b, may be compared to the basal eye-specks of the bulb of 

 Hippocrene. There are about eight or nine such larger pigment-cells, forming a crescent 

 at some distance from the opaque nucleus of the bulb ; and above this rises another row 

 of most elegant transparent pearl-like cells, c, encircling the former, which may be com- 

 pared to rudiments of as many tentacles. If this view of the structure of these eye-specks 

 is correct, each of them would correspond to one bunch of eye-specks, or one bulb, of 

 Hippocrene ; but of these bunches there would be eight in Tiaropsis, alternating with 

 the radiating tubes instead of being connected with their base, and, in addition to these, 

 a large number of simple tentacles with rudimentary eye-specks. I may be mistaken, 

 but I cannot help being deeply impressed with the diversity of structure which exists 

 among these lower animals, which is so great that the necessity of extensive comparisons 

 between these structures, in order to ascertain their homologies and relations, seems to 

 me almost of greater importance than among the higher animals; or, at least, as urgently 

 called for to advance our science. Again, is not a comparison between the clusters of 

 eye-specks in Hippocrene and the compound eyes of insects almost involuntarily called 



