THE ECHINUS, OR SEA-URCHIN. 
371 
The ambulacral system consists of a circular vessel round 
the gullet, with five pyriform csecal processes (termed polian 
vesicles ), and giving off five longitudinal vessels. Each of these 
longitudinal vessels passes up between the adjacent alveoli, up 
under the corresponding rotula, and down through the adjacent 
auricula, and finally ascends inside the middle of an ambu- 
lacrum, terminating blindly at its summit. Each longitudinal 
vessel gives off transverse branches, one opposite to each pair 
of pores, which become continuous with the saccular dilatations 
before mentioned at the inner ends of the tubular suckers. 
These ambulacral vessels are ciliated internally, and are kept 
in relation with the surrounding medium bv means of a 
straight axial canal, which ends above at the before described 
madreporic tubercle with its sieve-like perforations. 
The second or pseudh sexual system of vessels consists of two 
circular vessels (one round the rectum and one round the 
gullet) connected together by a dilated vertical vessel, said to 
be contractile, and also of two other vessels which accompany 
the alimentary canal. 
The nervous system has in its distribution much similarity 
with the ambulacral system. It is made up of a pentagonal 
ring, round the gullet, with five, hardly distinct, small ganglia, 
one at each angle of the pentagon. From each ganglion a very 
considerable nerve arises, and passing through the auricular 
arch and between the alveoli, runs up to one of the five eye- 
spots, where it enlarges and terminates. Each nerve as it 
ascends gives off lateral branches to the suckers. 
The nervous system is more superficial in position than are 
the vessels. Thus each nerve which runs up each ambulacrum 
is placed on the outer side of the similarly extending ambu- 
lacral vessel. 
The eye-spots are the only organs of sense known. 
The reproductive system is exceedingly simple. The sexes are 
in distinct individuals, and in each there are five pairs of glands 
— ovaries in the one case, testicles in the other. In each case 
they open externally by the apertures in the genital plates. 
The ovaries are a favourite article of food in Southern Europe ; 
they are of a bright orange colour when ripe. 
The process of development is extremely remarkable, and 
utterly different from anything met with in any of the typical 
animals yet described in the Popular Science Review. 
After the process of yelk division has taken place the ovum 
becomes an oval body, with an external, blastodermic layer, 
apparently made of coalesced particles of the results of yelk 
division. In this stage the embryo is covered over with external 
vibratile cilia. Soon a depression appears on one side of the 
body, and this is the future anal aperture of the larva. The 
