IM 



SEA-URCHINS 



SEAWEEDS 



middle of nn 'apical disc,' wliieh consist* of a 

 central plate Mirruumled by five 'ocular 'and five 

 ' genital ' plates. The ocular plates bear eye- 

 Bpecks ; tin- -I'liiul platen liear the openings of 

 the genitnl ducts, but one of the live i.- modified 

 as a n>a.lic|M,iic plate through which llni.l filters 

 and leaves the water-vascular system. From pole 

 to pole extend Ion meridians each a double row 

 of calcareous plates which fit one am .'her firmly. 

 Five of these meridians in line with the ocular 

 plates are known as ambulacral areas, for through 

 holes in their plates the locomotor tulie-feet are 

 extruded; the other five meridians, alternating 

 with the former and in line with the genital 

 plates, are called inter-ambulacra! areas, and bear 

 spines, not tul>e-feet. The Imses of the spines fit 

 over ball like knolis, on which they are moved 

 by muscles. There are also two peculiarly modi- 

 lied form- nf spines -the minute pedicellarhe, with 

 three HMppillK-bbldei mounted on a soft stalk, 

 and null globular sph:i*ridia which seem to be 

 sensitive to chemical changes in the water. The 

 pedicellariie have l>een seen removing pieces of sea- 

 weed and the like from the surface of the shell. 

 In front of (lie nuintli project the tips of live teeth 

 which work against one another, grasping and 

 grinding small particles. They are fixed in five 

 large sockets, and along with fifteen other pieces 

 form 'Aristotle's lantern,' a complex, somewhat 

 lantern-like masticating mill, of which Aristotle 

 took notice. The limy skeleton is formed in the 

 inesoilerni. and outside of it there is a delicate 

 ciliated ectoderm with a network of nerve-fibres 

 ainl some ganglion cells. Inside the shell, lining 

 the spacious Uidy cavity, there is again ciliated 

 epithelium. 



The nervous system consists of a ring around the 

 mouth, with a radial branch up each ambulacral 

 area, and of the superficial network. The tube- 

 feet, pedicellariie, and spines are all under nervous 



Fig. 2. Diagram allowing the Anatomy of Sea-urchin 



(after Hnxley): 



a, niiMilli : d. t-.-tli nf Atiitotln'* lantern ; r, muscles which work 

 tli* lantern ; '/. f'>d canal ; ' Hti of fi)Ad*4MMl ; / BMdnporta 

 pUt' i. tnne-canal ; , circular wnter-vnuwl ; <, radial wmfc-r- 

 YM*el i k, ampulla ; 1, tube-foot ; m, i-pine ; m, peillcelUrla. 



control, wiiil.- (he eve specks, the sph;eridia, and 

 tin- tu lie feet Adjoining the mouth urn especially 

 sensitive. The alimentary canal passes Uirmlgn 

 Arjsioilc's lantern, coils round the inside of the 

 shell, and ends in the apical disc. The body cavity 

 contains a lluid with Moating brown cells appar- 

 ently of some significance in respiration. 



The water-vascular system is of use in locomotion, 

 and perhaps also in excretion. From the apical 

 inadre|H>ric plate a ' stnne-eanal ' extends through 

 the body to a circular veaael round the upper end 



nf the lantern ; this circular vessel gives off live 

 radial vcs-el.-, from which the tube-feel are sup 

 plied, \\beii filled with fluid the tube-feet become 

 tense and are pressed against the MM face of the 

 rock: when the fluid flows back they adhere firmh, 

 and the sea-urchin drags itself tn\\anl- tin- attacii- 

 ment. There is a blood-vascular system, but it is 

 dillicult to trace. Inspiration seems to be dis- 

 charged in part by the body -cavity lluid, in part 

 by ten hollow outgrowths on the area round about 

 the month. 



The sexes are separate and resemble one another. 

 The reproductive organs are live spongy masses 

 lying beneath the apical disc. The eggl are fer- 

 tilised externally by spermatozoa wafted from 

 another sea-urchin ; and the free-swimming larva 

 out of which the adult develops, as in other 

 Echinoderms, by a remarkable indirect metamor- 

 phosis, is called a Pluteus, and has a yinim form, 

 a little suggestive of a many-legged painter's easel. 

 A few sea-urchins e.g. ilemiastcr carry their 

 young aliont with them among their spines. 



Most sea-urchins live oil' rocky coasts ; not a few 

 shelter themselves in holes in the rocks ; many 

 deep-sea forms are known. Most are very sluggish, 

 moving slowly by means of their tube-feet, in some 

 cases slightly" helped by their spines, in other coses 

 perhaps hindered. They feed in part on seaweeds, 

 mostly on organisms and organic matter found in 

 mud and other deposits. Many look as if they were 

 falling victims to their constitutional tendency 

 towards the deposition of lime, for there is hardly 

 any part of the iKxIy which may not become limy, 

 and in some of the flattened forms the body-cavity 

 is much restricted by cross beams of lime. 



Among other Echinodcrms, the Kchinoids may be 

 placed near Asteroids (starfishes) on the one hand, 

 and near Holothnroids (sea-cucumbers) on the 

 other, while they have without doubt affinities 

 with the extinct ('ystoids, which occupy a central 

 position among the classes. The sea-urchins are 

 often classified as follows : 



1. PALjai-EciiiNoinEA. Extinct forms, apparently with plastic 



slH'lls. occurring first in Lower Silurian nicks. 



2. DFSMOHTICUA Regular and symmetrical forms, such as 



Echinus, Btrongyloeentrotas, Cidaris (with very long 

 spines), Liiiidema ( with nuiiit-roii* dil!ii*<- <-\cs), Cyuno- 

 toma vrtn* (with poiMHiou* spines), Ectmmtliurkla- (with 

 flexible shells;. 



3. CLYI-KASTBOIDEA. Shield-shaped and often flattened forms, 



witli the end of the loud-canal outside the apical disc 

 e.g. Clypeaster. 



4. PETALOSTKHA. Heart-shaped forms, with excentrii' moiiih, 



without masticating organs, with the end ol ih. 

 canal away from the apical disc, with irregular anitm- 

 lacral areas e.g. Spatangus. 



The ovaries of Erhitins esctilrntiis nre sometimes 

 eaten, hut otherwise the sea-urchins hardly come 

 into touch witli human life. 



Seaweeds, a general and popular term 

 applied t<> a vast collection of lower plant-forms 

 growing on the gearoast from high-water mark 

 (or a little above that limit) to a depth of from 60 

 to 100 fathoms (rarely deeper), and all belonging 

 to the sub-class of the Thiillophyta. to which the 

 name Alga- lias been given (see Al.c.l.). Any 

 detailed treatment of the organisms included under 

 the term would ! impossible within the limits of 

 a short article like the present. It must suffice, 

 on the one hand, to indicate the chief variations in 

 structure and life history of typical representatives, 

 and on the other to refer briefly to the more im- 

 portant points in the distribution and classification 

 of the group. The short bibliography appended 

 will supply a guide to the very extensive literature 

 of the subject. 



An examination of an average shore-telt exposed 

 at ebb-tide reveals the fact that this area may be 

 roughly divided into four regions: (a) a zone at 

 and above high-water mark, characterised by the 



