596 



CRUSTACEA 



hatched young is unlike, often very unlike, its 

 parent. Even when there is no metamorphosis 

 after hatching, traces of transformation, as opposed 

 to continuous development, are sometimes to be 

 detected in the earlier history while the embryo 

 is still within the egg-case. (a) The crayfish 

 ( Astacus ) has a very much abbreviated life-history, 

 for the newly-hatched form is almost quite like 

 the adult, (b) The newly-hatched lobster (Hom- 

 arus ), however, begins life a little further back, 

 in what is known as the Mysis stage, in which 

 the thorax bears two-branched swimming append- 

 ages, (c) Most other higher Crustaceans (e.g. 

 crabs ) begin at a still lower level, in what is called 

 the Zocea stage, with a short unjointed thorax and 

 a segmented abdomen without limbs, (d) The 



Development of a Prawn (Penceus} : 

 a, Nauplius ; b, Zosea ; c, Mysis ; d, adult 



Decapod Penseus, a shrimp-like creature, has its 

 life-history still more drawn out. It quits the egg 

 as a Nauplius, an unsegmented larva with three 

 pairs of appendages, the first unforked, the other 

 two pairs double-branched. These correspond to 

 the first three appendages of the adult. The median 

 eye is also a distinctive feature in the Nauplius 

 larva. The Penaeus Nauplius has with successive 

 moults first to become a Zosea, and then a Mysis, 

 and then an adult. It is as a Nauplius that the 

 majority of lower Crustaceans leave the egg, but 

 then they do not climb so high. To understand 

 the circuitous life-history of a form like Penseus, 

 we have to note that it begins in the Nauplius 

 stage, at the level of the lowest Crustaceans, and 

 gradually climbs through a series of higher and 

 higher stages, each of which is represented per- 

 manently by some division of Crustaceans which 

 have not risen higher. If the various grades from 

 Nauplius up to Decapod adult represent successive 

 historic levels, now exemplified in the classification 

 of Crustaceans by those which were left behind at 

 each lift, what the Penreus does is to recapitulate 

 in its individual life-history the historic evolution 

 of the class. This idea has been beautifully applied 

 to Crustaceans in Fritz Miiller's Facts for" Darwin. 

 The various grades of Nauplius, Zotea, Mysis, and 

 adult Penreus (overlooking intermediate ones) 

 may be compared to stations which mark the 

 gradual extension of the Crustacean line of advance. 

 Penseus has to travel along the rails laid down by 

 the ancestral history, and has to stop for variable 

 periods at the successive stations between the 

 starting-point and the terminus. Crabs skip over 

 the Nauplius station, and like most other Decapods 



start at the Zosea point ; lobsters abbreviate still 

 further, and begin as Mysis forms ; the crayfish has 

 found the shortest cut of all. Some of the lower 

 Crustaceans never get far past the Nauplius stage, 

 while others remain practically on the Zoaea grade. 

 The life-histories of Crustaceans vividly illustrate 

 how the individual life-history is a rehearsal of the 

 historic evolution of the kind, or more technically, 

 how ontogeny recapitulates phytogeny. 



Habit of Life. The acorn-shells fastened to the 

 rocks, wafting in their food by their curled feet ; 

 the barnacles moored to floating logs and ship- 

 bottoms ; such extremes of parasitism as are illus- 

 trated by Sacculina on hermit-crabs ; the hermit- 

 crabs themselves, stealing the shells of Gasteropods, 

 or entering into partnership with sea-anemones ; 

 the thousand minute and active water-fleas ; the 

 wood-lice, quite terrestrial ; the brine-shrimps in 

 the salt-pools ; the fresh-water crayfish ; the giant 

 marine lobsters ; the land-crabs, habituated to in- 

 land life, sufficiently suggest how varied are the 

 habits of Crustaceans. Some Crustaceans form 

 masking shelters for themselves out of Tunicates, 

 or get covered over by a concealing growth of 

 seaweed, sponge, hydroids, &c. A few forms 

 are known to make a stridulating noise. The 

 general intelligence of the class is probably con- 

 siderable (see CRAB). On the whole the mem- 

 bers of this class are active animals, but on each 

 side of the medium activity of the majority there 

 are extremes. Thus, not a few active marine 

 forms are phosphorescent, while parasitism ( to the 

 extent of some 700 species ) occurs in most of the 

 subdivisions. Many of the parasites are very 

 striking in the contrast between the free-swimming 

 young and the ne plus ultra of degeneracy in the 

 adults (see DEGENERATION, PARASITISM). Some 

 of the interesting cases of Commensalism (q.v. ) 

 have been referred to under that title ; while some 

 of the external parasites show in the castration, 

 &c. which they effect on their hosts, how real in 

 such cases is the direct influence of the animate 

 Environment (q.v.). The diet of Crustaceans is 

 very varied ; the majority are carnivorous and 

 aggressive ; many feed on dead creatures and 

 organic debris in the water ; others depend largely 

 upon plants. They often lose limbs in fighting or 

 otherwise, and have the power of replacing what 

 they have lost. 



Classification (after Glaus). (A) Entomostraca. 

 Lower forms, small, simple, with variable number 

 of rings and appendages, with not more than three 

 appendages concerned in mastication, usually leav- 

 ing the egg as a nauplius. ( 1 ) Phyllopoda : ( a ) 

 Branchiopoda e.g. Biine-shrimps (q.v.), Apus, 

 Estheria, &c. ; (b) Cladocera the common 'water- 

 flea' Daphnia, Moina, &c. (2) Ostracoda com- 

 mon Cypris (q.v.), Cypridina. (3) Copepoda 

 common Cyclops (q.v.), Lernsea, &c. (4) Cirri- 

 pedia Acorn-shells, Barnacles (q.v.), Sacculina, 

 &c. (B) Malacostraca. Higher, larger forms, 

 with nineteen segments, with more than three 

 appendages concerned in mastication, usually 

 quitting the egg at a higher level than the Nau- 

 plius : ( 1 ) Leptostraca, Nebalia, a primitive form 

 with bivalve shell ; ( 2 ) Arthrostraca, with free 

 thorax, and no cephalothoracic shield, eyes ses- 

 sile : (a) Amphipoda Sand-hoppers (q.v.), Cap- 

 rella, &c. ; (b) Isopoda Wood-lice (q.v.), Asellus, 

 Tanais, &c. ( 3 ) Thoracostraca, with all or part of 

 thorax fused to head, and with a cephalothoracic 

 shield, eyes mostly stalked : (a) Cumacea, sessile- 

 eyed Cumaj (b) Stomatopoda, with gills on ab- 

 dominal feet Squilla ; (c) Podophthalmata, with 

 stalked eyes and large shield: (i) Schizopoda, 

 with eight pairs of double thoracic feet Mysis; 

 (ii) Decapoda, with thorax fused to head, and 

 last five thoracic feet not double ; long-tailed 



