9. HmhTyo of Aslacusjliiviat ilis ^\i&t helore leaving the egg. Tlie carapace has been cut away from tlie side 

 turned toward the observer, in order to show tlie gills. The first and last pairs of abdominal appendages 

 are undeveloped ; otherwise the embryo has the essential characters of the adult, and undergoes no marked 

 metamorphosis after leaving the egg. From Kathke, Untersueliungeu iiber die BUduug uml Kntwickelun" 

 des Flusskrebses, Taf. I. fig. 16, Leipzig, 1829. 



10. Astacus Jluviatilis, two lately hatched young attached by their ehelipeds to one of the abdominal appendages 



of the mother. Four times natural size. 5, protopodite ; e, endopodite ; f, exopodite of the abdominal 

 appendage of the mother, tj, ruptured egg-cases. From Huxley, op. cit., p. 41. 



11. Spermatozoon oi Homarus Americanus. From a drawing by Faxon. 



12. Emhryo o{ ffmnarus Amcriamus. VIII, third maxilliped. The dotted line rests on the e.xopod. The lono'er 



endopod extends beyond the tip of the exopod. The endopods of the succeeding pairs of appendages are 

 concealed by the exopods. h, heart. From a drawing by Stiuipson, June 6, 1852. 

 13- 16. From drawings by Faxon, Newport, R. I., July 18, 1881. 



13. First antenna of embryo of Homarus Americanus just before hatching. The shaded part indicates the 



antenna of the larva seen through the cuticle of the embryo. 



14. Second antenna of the same. 



la. Tail of the same, d, median spine of the tail of the first larval stage, i, lateral spine of tail of first larval 

 stage. All the spines of the enclosed larval tail are shortened by invagination. 



16. Tail of first larval stage of the same. The larva is about to moult, and the tail of the following larval stage is 



seen through the cuticle. 



17. First larval stage of i7omarus^?Hfn«m?(s. Leaves the egg in the My.sis condition. Natural size, about 8 mm. 



long. From a drawing by A. Agassiz, Nahaut, Mass., July 1, 1866. 

 18-30. Development of Pagurida;. 



18. Section of egg of Eujxigurus Prideauxii befoi'e cleavage. The nucleus has (livided into eight, four of which 



are seen in the section. Each nucleus is surrounded by a thin layer of protoplasm which sends out tliread- 

 like processes into the surrounding yolk. The segmentation is at first total, but after the fourth phase the 

 cleavage spheres fuse in the deiitoplasmic centre of the egg, and the subsequent cleavage is superficial. 

 From Mayer, Zur Entwicklungsgesehiehte der Dekapoden. Jenaische Zeitschr., XI., Taf. XIII. fig. ], 

 1877. 



19. The same after the fourth cleavage, n c, nuclei surrounded with a layer of protoplasm. From Mayer, op. 



cit., Taf. XIII. fig. 4. 

 20-23, 25-30. From drawings by Faxon, Newport, R. I., Angust, 1881. 



20. First larval stage of Pagurus. Leaves the egg in the zoea form, the first and second maxillipeds serving as 



locomotive organs, the third niaxillipeds (VIII) present but rudimentary. No thoracic or abdominal 

 appendages. The sixth abdoinin.d segment is fused with the telsou. The posterior thoracic segments are 

 potential merely. 



21 . First antenna of the same. 



22. Second antenna of the same, s;), spine. ?•», rudimentary flagellum. re, squamiform appendage. 



23. One half of the hind border of the tail of the same, armed with seven seta;, the sixth of wdiich (counting from 



inner side) is reduced to a small curved hair, "\\4thin the tail, represented by light shading in the figure, 

 are seen the caudal setae of the next larval stage. It appears that the inner seta of the first stage will be 

 replaced by two (1', 1) in the second larval stage. 



24. Tail of emhryo of Enpogu.riis Pridmuxii. The seta numbered 6, which becomes a rudiment in the first larval 



stage, is well developed. All the sets are feathered except the outer ones, 7. From Mayer, op. cit., Taf. 

 XV. fig. 43. 



25. Tail of second larval stage of Pagurus, from Newport, R. I. Comparison with Fig. 23 shows that a new seta (1') 



has been developed on the inner .side of the seven primary seta; of the first larval stage. 



26. Moutli parts of the same, lb, labrnm. m t, meta.stoma. Ill, mandible. IV', IV", IV", first maxilla. 



V, V", V", V"", second maxiUa. 

 27- Third lar\'al stage of Pagurus. The exopods of the third maxillipeds have become functional swimming 

 organs. Rudiments of the ehelipeds (IX) and two or three following pairs of thoracic appendages have 

 appeared, and they are simple from the time of their first appearance. There is thus a syncopation of 

 the Mysis stage in Pagurus and in Anomoura generally. In the sujipression of the Mysis stage and in 

 the late functional development of the third pair of maxillipeds, tlie Anomoura resemble the Brachyxira 

 rather than the typical Macroura like Pala;mimctcs vulgaris (see PI. XL). In the structure of the second 

 antenna;, spatulate form of the tenninal segment of the abdomen, and the appearance of the posterior 

 abdominal appendages (XI.X) in advance of the rest, Pagurus agi'ees in its development with Palamonctcs 

 mtJgaris. XI. \, last pair of abdominal limbs. Their inner branch is comnu^ncing to grow as a small lobe 

 from the proximal end. The sixth segment of the abiUunen is now a free segment. 



28. First antenna of the same. 



29. Second antenna of the same. 



30. Telson and appendage of sixth abdominal .somite, ae, rudimentary inner branch of appendage. The Arabic 



numerals indicate the correspondence of the setse of the telson with those in the earlier larval stages. 



