168 Papers from the Department of Marine Biology. 
of the invertebrate animals of the deep sea, due possibly to the fact 
that the red rays from the sun are rapidly absorbed as they penetrate 
into the depths of the ocean, so that at depths greater than 1,000 
fathoms there is practically no red light, and here red-colored animals 
must appear black and be well concealed in the general darkness of 
their abysmal realm. The Coronate are characterized by having a 
deep ring-like furrow cutting into the thick wall of the sides of the um- 
brella; and peripheral to this are gelatinous thickenings in the radii 
of the tentacles and sense-organs. The gelatinous wall of the bell is 
much reduced in thickness at the ring-furrow, the thin part acting as a 
hinge to permit the creature to close its bell during contraction. These 
forms are common in the deep waters of the Philippines, as indeed they 
are upon the bottom of all seas 500 fathoms or more in depth. Most 
of the species, such as Atolla and Periphylla, do not normally come to 
the surface, but a closely allied form, the little Nawsithoé, is one of 
the most universally distributed surface forms of all warm seas, while 
the mature Linuche occur in vast numbers, all rapidly pulsating and 
resembling little brown thimbles, the swarm often covering square 
miles of ocean in tropical regions. 
IV. The Semeostomata are the common large jellyfishes of our coasts, 
such as Aurellia, Cyanea, and Dactylometra. They have tentacles and 
a single cruciform mouth provided with veil-like lips, and there is no 
ring-furrow cutting into the surface of the umbrella, such as is seen in 
the Coronatze. They are common in bays and brackish estuaries and 
are the largest and most conspicuous of all jellyfishes in temperate 
regions. 
V. The Rhizostome are the large jellyfishes of the East Indian and 
tropical seas, where they are commonly as conspicuous as are the 
Semzeostomez in our cold northern waters. They have numerous 
mouths and no tentacles, and their gelatinous substance is often as rigid 
as newly formed cartilage. Often they occur in swarms in the harbors, 
swimming strongly against the tide, and after a storm great numbers 
are found stranded upon the shore. The rigid gelatinous substance of 
these jellyfishes is very characteristic and was seen in fossil forms found 
in the Jurassic lithographic slates of Steinheim in Bavaria, showing that 
in the age of the Reptiles these most highly differentiated jellyfishes 
existed. Indeed, so old are the jellyfishes that their relationship to the 
echinoderms, sponges, and ctenophores remains unknown—all inter- 
grading forms, if such ever existed, having died out long ago, as is often 
the case in very ancient orders. 
In an early stage of development the higher animals commonly pass 
through a condition in which they have only an outer cell-layer and a 
cavity lined by cells destined to form the intestine. Theoretically 
speaking, they are simply little 2-layered sacks, the outer layer being 
the external skin with its nervous and sensory organs, and the inner 
