754 



REPORT — 1884. 



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In the Penaidea the youn<^ is supposed to be hatched in a naiiplius form, which 

 has been demonstrated in Leiicifir: in Serijestcs and PeiKtus, as has approxiniatelv 

 been shown in Sergestca, it is produced in the form of a blind Xehiphocftn'si. In tbiv 

 ■division the ova otb not connecttd with the animal by any membranous nttacli- 

 ment, and iire probfibly depo>itfd in the open sea, there to be brou|;ht to matuiitv 

 if they do not fall a prey to liungry animals. 



^Vithin each division a ^'reater or less departure from the typical cliaractcr 

 takes place, the variation generally bein<j the frreater or less developed condition 

 of the Inancliife and the pereiopodaor walking legs, and this departure is geiierallv 

 connected with an important change in some external feature belonging to tl^ 

 animal, alt hdugli there are some instances, as may be seen in tlie Knvopean and 

 N(n'th American crayfish, where there is an alteration in the branehial ciiinlitioii 

 Avitliout any external evidence. Sometimes, as in Xotosfomus and Micrsia the 

 branchial arrangement continues the same, while the external features vary ctin- 

 siderably. Again, in the history of development, as in the genus Al/)/ifniii,ty 

 young of some species are hatched in a distinct form from that of others, while :!!•■ 

 parents exhibit no sign of variation, while again the common shrimp and tln' 

 common prawn (that is Vra7igon and I'dlcfmun), which evidently diiler fromcacl; 

 other, liatcli tlieir young in the same form. 



The variability according to its importance and constancy breaks tlie several 

 • divisions into Fainiiics, Genera, and Species, while the character nf developm-iii 

 demonstrates a continuity in the history tbat exists in apparently widely separated 

 species; an example of which may be seen in comparing the genus 7/wce»s witli 

 that of I'ti/iniirKs, which appear to possess little or nothing in common, while their 

 offspring in tiie earliest stages of development appear to be identical. 



J(Ut in whatever stage the young may quit the ovum, there is one comiuori 

 chapter in their history ; that is, they invariably seek the surface of the sen, 

 feeling as it were for light, air, and warmth, wliich for most of them would 

 fippear to be the chief condition for which they struggle in the earlier days of tl.eir 

 existence, after which they generally sink to that abyss, whence they have bten 

 brought to light through the far-reaching powers of modern investigation. 



Animals so delicate and fragile as these are very liable to injury and destructijii 

 from many causes. 



As far as our experience teaches us, they never in the rougher weather reach 

 the surface, or, at least, remain there, and where they have the power of cougiv- 

 gating most they fall a prey as food to many larger animals. Talung their several 

 means of destruction besides those that seem natural to the condition of their youii:,' 

 lives, we must suppose that a very much larger number must be hatched as com- 

 pared with those that permanently arrive at the adult form. 



It is ditlicult to determine the number of young born of any known sp(>cies, hui 

 it must be very great if we consider that a prawn of average size bears about a 

 thousand eggs, and that a large Palinurus, such as is sold in our mjirkets a.s \h 

 crawlish or spiny lobster, produces many thousands ; the ova being small, or aL"iit 

 one-fiftieth of an inch in diameter, while those oi Ilomarus are one-tenth, and those 

 of Astacus are one-eighth. 



The stage at which development of the embryo has advanced at the time when 

 it quits the ^^^ appears to bear some relative proportion to the size of the ovum. 



Thus our common edible shrimp has an i^^^r only one-fortieth of an inch i'l 

 diameter, while that of the Arctic species has one that is the eighth of an inch. 

 The smaller ovum produces an animal in the zoaja condition, while from the larger 

 the young appears in the megalopa stage. 



Palinurus, Ardm, Ihaccus, and other allied genera, produce their y(nuig in an 

 intermediate condition, more advanced than a zonsa and less mature than a megii' 

 lopa, in a form long believed to be an animal of distinct individuality known a^ 

 Phyllosoina. 



The genus Palinurus, perhaps more than any other, will assist in demonstratin;: 

 the bearing which the character and condition of the young have in the geographi- 

 cal distribution of species. 



I cannot here resist pausing to remark that, if an animal that has furnished 



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