and Anatomy of the EcMnodermata. 375 



4. Summary of Results, with a Description of the Principal 

 Conditions of the Anatomical Structure of the Sea-urchins. 



In giving the following description of the anatomical and 

 histological structure of a Sea-urchin, I indicate only the 

 principal results which seem to me to be of importance for 

 the comprehension of the Sea-urchin's body. At the same 

 time I do not refer at all to the skeletal characters, seeing 

 that these are already sufSciently known and investigated, 

 especially through the works of Lov^n and other naturalists. 



In the Holothuri^, which are destitute of any spines or 

 similar structures, I have been able to describe the sense- 

 organs situated in the skin. In the Sea-urchins these are 

 nearly all (with the exception of the tentacles) placed upon 

 stalked organs, the pedicellarice. By this means an efficiency 

 is secured to them which sense-organs on the skin could not 

 develop on account of the frequently very long spines. 



On the pedicellariee, with their three-valved forceps, the 

 mechanism of which I have described in detail, supposed 

 sense-organs were observed by Sladen only in one form, the 

 so-called gemmiform pedicellari^ ] but neither that naturalist 

 nor Kohler succeeded in demonstrating nerve-terminations. 



Exquisite sense-organs occur in all pedicellariee — gemmi- 

 form, tridactyle, and trifoliate. Special tactile eminences^ 

 often of complex structure, occur on the inner surface of the 

 valves ; these are beset with rigid setas. Nerve-branches 

 run to these tactile eminences. In general three nerve-cords, 

 composed of the finest nerve-fibres and ganglion-cells, were 

 observed ; these pass into the capitular part, and while each 

 gives oif numerous lateral branches to the musculature, sense- 

 epithelium, &c,, they could be traced to the tip of each valve. 

 The glandular sacs in the wall of the pedicellarige are of par- 

 ticular importance in the seizing of any objects ; whether 

 they exert a paralyzing action upon smaller animals, such as 

 worms, is still to be ascertained. 



Following on these organs come the glohiferi, newly 

 discovered organs which serve as weapons. They occur only 

 in a few genera. As further appendicular organs of the skin 

 Lov^n's remarkable sphceridia are to be mentioned. At their 

 base may be found a nerve-ring of the same structure as that 

 which is detected on the spines. From this basal nerve-ring, 

 which shows itself externally by a thickened epithelium, an 

 epithelial pad, nerve-fibres run sometimes to the musculature, 

 sometimes running to the apex of the spine in the four, five, 

 or more long ciliary bands. Similar nerve-structures occur 



